One bit of bad advice you'll frequently hear is to install a fuel filter in your fuel line between the fuel pump and the carb.
Don't do it.
The added mass of the filter combined with the vibration of the engine and road serves to wiggle the brass ferules out of the carb and fuel pump. When that happens if you happen to have some marshmallows with you, fine, otherwise there's little you can do but stand by and watch it burn.
Volkswagen used untreated mild steel for their fuel tanks. Because of the age of the typical Volkswagen, the fuel tank and fuel lines are generally quite rusty; that is the source of the residue you find in the carb bowl. Adding another filter (you already have two) deals only with the symptom, the real solution is to tackle the rust itself.
The factory service manual addresses fuel tank refurbishing in considerable detail but the real secret to success is what you do after you get rid of the rust, which is to use a chemical sealant commonly called 'sloshing compound.' You pour it in and slosh it around, then allow to dry. Be sure to remove the strainer first. This is some very tough stuff, used in metal fuel tanks on aircraft. It is available from J. C. Whitney.
A rusty fuel pipe is a more serious sort of problem since the location of the pipe within the central hump makes replacement difficult, best done when the body is removed from the chassis. I've mentioned another fix, the use of an externally routed replacement fuel line, in one of my earlier articles ('The Stainless Steel Craftsman').
Cleaning your existing filters and even adding another can buy you a little time but please do not install it within the engine compartment. Under the fuel tank is the safest place, followed by under the rear seat deck (ie, near the nose of the transmission). Veedubs love to burn; they're very good at it.
Your fuel system already has two strainers installed, one in the fuel tank, the other in the fuel pump. The latter is often overlooked as a maintenance item. On the late model pumps it is under the top cone, in early pumps it's behind the big brass nut. When you have a rusty fuel system you should clean the fuel pump strainer when you do your oil change. (But be careful, remember the fuel will flow by gravity once the system is opened.)
Rust forms in the fuel system due to an accumulation of water vapor from the atmosphere. When it condenses it collects in the lower-most stampings of the fuel tank, a depression around the fuel outlet fitting, where it produces pin-hole leaks. VW fuel tanks are not made of terne-plate (lead-coated steel normally used for tankage by American auto manufacturers) but are common mild steel sheet. Once liquid water is present in the fuel system it's difficult to remove without draining the tank through the filler neck (ie, up-ending the thing). As a general maintenance item the usual method is to add a 'dryer' to your fuel, as frequently as dictated by your local climate. Gas 'dryer' is nothing more than wood alcohol (methyl methanol?) and is available at any auto parts store. Being hygroscopic, the alcohol mixes with the water and, if there isn't too much water, will be burned as fuel.
The water/rust problem is less frequently seen on veedubs fitted with the full array of pollution control devices, since the fuel tank is not vented directly to the atmosphere.
If your fuel tank is seriously rusted, the wiser course is to replace it rather than repair it. (I'll pause here and wait a minute until all the weldors stop rolling on the floor with laughter.) Replacement fuel tanks are available although the workmanship is rather shoddy; they often arrive already rusty. If you buy one it's a good idea to treat it as if you'd made it yourself, removing the existing paint, smoothing up the welds (bloody dangerous!) and repainting it with a high quality epoxy. The interior should be sloshed as a matter of course. Serious rebuilders have their new or repaired fuel tank powder-coated, which bakes the enamel to both the interior and exterior surfaces. The super-serious (and wealthy) have a new tank fabricated from stainless steel or aluminum.
Working on the fuel tank is one of the easier tasks of VW maintenance (at least, on a bug :-) since it is so accessible, held down with just four clips&bolts, and at waist level. (This isn't true for '68 and later models; the filler neck is especially difficult to re-seal.) Be sure it's empty before working on it -- six pounds per gallon can make a heavy load -- and that you have new fuel line on-hand. The existing fuel line under the tank will probably break like a stick when you try to disconnect it. (Lift one side of the tank, peek under, reach down and wiggle it loose.)
Pulling the fuel tank also gives you an opportunity to remove the fifty pounds of sand & gravel that accumulates on the 'smuggler shelves' behind the wheels when you take the short cut between San Ignacio and La Parisma.
Copyright © 1995 Robert S. Hoover
Wednesday, December 6, 2006
AV - Yo! Fuel Tank!
Riveted aluminum fuel tanks are smart. They're easy to build and superbly practical for the homebuilder since she can make them in whatever shape she needs.
Most folks shy away from this fabrication technique due to the high cost of Pro-Seal, still listed at more than $8/oz in the Aircraft Spruce catalog (P/N 09-38500 "2oz sealant" $17.85). But now that Thiokol's patents on polysulfide sealants have expired seam-sealers that do equally well are available for pennies per pound instead of dollars per ounce. Life Industries is one such source. They make a line of polysulfide sealants for marine applications, including a two-part fuel-proof formulation used to calk fuel tanks & bilges.
With aluminum, the sealant-bonding question - getting the stuff to stick - is a no-brainer. Go to your local Home Depot and buy a quart of JASCO ‘Prep & Primer.' Or buy a quart of ‘AlumaPrep' from Aircraft Spruce. Same stuff, chemically speaking. Of course, the aviation-grade' alumaprep is dramatically more expensive.
Degrease then etch the panels you want the sealant to adhere to for thirty minutes in a solution of ‘Prep & Primer.' (The strength of the solution isn't critical. Anything from 1:1 to 3:1 works fine on clean aluminum. If using Alumaprep, follow their dilution instructions.) ‘Prep & Primer' is a phosphoric acid etchant made specifically for galvanized and aluminum surfaces. Scrub the etched surface with a Scotch-brite pad and neutralize with boiling water. The result will be a matte white finish.
To insure greater integrity of your rivet line, you may wish to use countersunk rivets. The dimple adds depth to the rivet line, making it stiffer without increasing its weight. The 120 degree dies you need for poppers are available from Airparts in Kansas City (www.airpartsinc.com) for about six dollars. And from other folks, too. The dies are used with your regular pop-rivet gun. Of course, if you have a lathe it takes only a few minutes to make such a set of dies and even un-hardened they'll last for several plane's-worth of dimples.
If you've never used flush-head poppers, run a few rows of sets before tackling the tank. The dimpling process enlarges the hole. The geometry here is subtle so be cool, work at the pilot-hole level, opening up the hole to rivet-size after you've dimpled & fitted the row. If you don't, the rivet will be too loose to pop; it'll just pull out. Be very careful when deburring as you'll be working on a corner instead of a flat. A file may be a better choice than a regular deburring tool. (These factors have probably contributed to the Conventional Wisdom that sez flush-head poppers don't work very well. They work just fine, but only when they fit the hole.)
With a pitch of about an inch aluminum poppers provide more than adequate strength for this application. Indeed, the stiffness of steel flush-head poppers dictates a minimum metal-depth of about forty thou. Anything thinner and you're liable to pull the popper through the hole before it can form a large enough shop-head to snap the mandrel.
You can buy flush-head aluminum poppers from J.C.Whitney in boxes of 500 for about $12. (JCW item# 14xx4090A, box of 500, $12.19) The short ones do fine for this type of job.
As with all poppers, be sure to wash them good in MEK prior to use. The manufacture of pop rivets always leaves some amount of lubricant on the finished product. That tiny trace of oil will interfere with the adhesion of the sealant (and of your zinc chromate, when using steel poppers on your other panels).
Steel poppers may be a wiser choice for attaching the flanged aluminum fitting for the tank's outlet. You may of course use steel button-head poppers for the entire tank if you wish. I like the flush-head aluminum jobbies for the seam-lines because they give me a stiffer joint at less weight.
If you prefer to use solid rivets you'll need to provide access for bucking the things. Wag Aero still sells sealed blind-nuts for a reasonable price (Cat# L-676-000 Pkg of 50 for $10.95) Sealed blind nuts are standard for fuel tanks. The threads of the screw are sealed away from the contents of the tank inside a little dome. Slosh the tank, you can still remove the access panel.
Tanks tend to violate the rule for panel size vs edge support so you'll probably want to pound an ‘X' bead into the four sides and the bottom. If the top of the tank is curved it will already be stiff enough but the sides & bottom will tend to be pretty wimpy, especially if you're using soft aluminum. (Almost anything will do for making a tank. Don't tell anyone but I've made tanks out of siding aluminum. )
If you don't understand what I'm talking about here, take a look at a steel Jerry can. Some guys like to roll such flutes into the panel but you can do perfectly well by making up a suitable groove or gap in a board, laying the panel across it and making several light passes along the groove with your rubber mallet.
Fuel tank is usually an irregular box. Occasionally an irregular cylinder. Cylinders support themselves but boxes don't. If the thing has corners, plan on adding a couple of baffles, not only to control the slosh but to stiffen the structure.
Rivet-on flanges for the filler and outlet are available from aircraft suppliers but since they are simple turnings they are easy enough to make if you have a lathe. And even if you don't. There are thousands of hobby-machinists on the Internet, their weapon of choice a little 7x10 lathe that's plenty big enough to whip out a set of fuel tank fittings. To track down such folks just go to the appropriate Newsgroup – rec.crafts.metalworking is but one of dozens of such groups – post a message having ‘Help!' as the subject line, describe the job and tell them where you're located. The squeaking wheel gets the grease - keep shouting until you connect with someone in your area. Like all machinists - which is what these folks are... the size of the machine has nothing to do with it - he'll need an accurately dimensioned drawing to work from and you'll probably need to provide him with the stock. Applying sealants is messy as hell, especially Pro-Seal and the other polysulfides. Masking off the area to be sealed/riveted will help and you might want to consider PK's instead of clecos. Polysulfide sealant is close to the perfect adhesive, it'll stick to anything... and doesn't like to come off. (For dimpled holes you'll need the longer (ie, 3/8") PK's.) Grubby-up a PK, throw it away. You're out maybe two cents.
Give the surface to be sealed a final wipe-down with MEK (or whatever solvent is recommended for you sealant). Allow it to evaporate. Apply the sealant according to the instructions. Most call for a smooth, uniform coating on both surfaces. Not too thick, a few thousandths is all you'll need if your rivet-line is a good fit. And not too wide, about three-quarter of an inch, max. Most of this stuff cures by reaction with water vapor in the air, something present everywhere on our particular planet. Cure time is a function of the width of the sealant-line and the humidity in the air. (That's why the stuff is so popular with boaters – it cures underwater faster than out of it.) The two-part formulation cures faster than the no-mix stuff.
You only need about three ounces of sealant for a ten gallon tank, most of which will go on the flanges, your tools and your clothes. (If you've never used Pro-Seal before, buy a pint :-)
If you use the 2oz kits, the little tubes can be hard to handle without the matching gun, a $75 item. (Aircraft certified, right? :-) A dime's worth of Bondo will allow you to modify a regular calking gun to accept the 2oz aircraft-certified cartridges. If you want to go that route, I'll tell you how to make an adapter.
Aircraft Spruce also lists the stuff in pints [$37] and quarts [$74] but the secret to using bulk-packaged sealants is how to mix & apply the stuff without gluing yourself to the wall. The usual mix ratio is 10:1 and is fairly critical. The use of a ratio'd balance beam, baggies for one component and a Teflon cup for the other is a fairly common procedure. Once you've balanced the beam, pour the One-stuff from the Teflon cup into the Ten-stuff in the plastic baggie then seal up the baggie and mix the stuff by squishing the baggie, like colorizing oleomargarine in days of yore. (That's my yore, not your yore.) Once the color is uniform, snip a corner of the baggie and squeeze the stuff out like decorating a cake, using a scrap of metal as a palette knife to smooth the bead to a uniform thickness across the bond line.
Standard practice when using poppers with a sealed structure is to dip the degreased popper in the sealant just before you stick it in the hole and give it a little twist. Don't get too far ahead with the sticking & twisting before coming back and doing the popping. The structure should be perfectly secured with a PK in about every fourth hole giving you three poppers in a row. When you pop, always do the middle one first. Once it's popped some guys butter a smear of sealant into the mandrel hole but it's not necessary if you're going to slosh the tank.
After your tank is fabricated leave it alone for about three days, until the sealant is cured. After it has cured you can provide yourself with virtual 100% leak-free assurance by sloshing the tank with a PVA fuel tank sealant. J. C. Whitney will sell you a quart of the stuff for about thirty bucks. You need less than a pint but I haven't found anyone who'll sell me that small a quantity. As with the polysulfides, there are only a few companies that make fuel tank sealants and most use functionally identical formulations. The thirty dollar stuff from J.C.Whitney appears to be the same as the hundred dollar stuff with ‘aircraft-certified' on the label. Maybe it's not but it works the same.
Since your tank was already etched, the sloshing sealant is going to form a perfect bond. Plug the outlet, pour in the sloshing sealant (it's a creamy white stuff; the vehicle is MEK) seal up the inlet (I use a hose clamp and piece of inner tube) then commence rolling the tank over and around and up and down... but in a logical fashion. What you want to do is to flow the sloshing sealer over every part of the interior surface.
After sloshing the tank, drain the sealant back into its can and seal it up good. Remove whatever is plugging the drain so air can circulate through the tank, prop it so it can drip out then leave the thing to cure.
Takes about 24 hours.
When it's cured, get your light wand and your bore scope and whatever else you need and inspect the interior surface. It should have a uniform white coating.
The sealant is a form of PVA – polyvinylalcohol. Once cured, it is impervious to virtually all solvents, including gasoline, alcohol and water. I've used the J.C.Whitney stuff on steel, aluminum and fiberglas with excellent results (JCW "Alcohol resistant Gas Tank Sealer" item# 12xx8316Y each $28.99). How well it works depends largely on how well you've prepped the surface. Basic rule is to have it perfectly free of grease, including fingerprints. For aluminum, you need to provide some ‘tooth' to the surface, which is accomplished by the etchant (ie, the Prep & Primer stuff).
Lots of ‘expert' homebuilders damn such tanks with faint praise. Sure it works... but real fuel tanks are always welded, yadayadayada... Sure they are, Mr. Expert. (I invite those experts to join me at FlaBob as we refurbish a P-51... and its riveted, sealed and sloshed fuel tanks.)
Your fuel system should have a strainer in the tank. Smartest one you can get is to MAKE YOUR OWN using a short length of 3003 aluminum tubing, slit about every quarter inch with a hobby saw. The typical hobby saw leaves a kerf about .028" wide, much smaller than the mesh of a cheap finger strainer. Cut enough slots to insure the thing will flow enough fuel.
The tank should be equipped with a shut-off valve. See the Northern Hydraulics catalog. Go to the same source for your gascolator. It ain't aviation-certified but it works and doesn't cost the earth.
Basic fuel tank plumbing is to keep a constant down-ward flow of fairly large diameter tubing from the tank to the gascolator. The idea here is that anything large enough to get through the strainer in the tank will not block the fuel line but will simply end up in the gascolator.
If you're flying a VW, a primer makes for easier starting. Since you hand-prop real engines, put the primer near the gascolator; no need to put any more fuel in the fuselage than absolutely necessary. Great Plains sells a good primer at a fair price.
For lo-buck builders, riveted, sealed and sloshed fuel tanks are a practical alternative to other methods of fabrication, their use so common we tend to forget others may not have heard of them.
-R.S.Hoover
PS - The ‘xx' in the JCW item numbers is different for each catalog but the basic number stays the same.
(Ed.Note: Originally posted to the internet in Oct. 2003)
Most folks shy away from this fabrication technique due to the high cost of Pro-Seal, still listed at more than $8/oz in the Aircraft Spruce catalog (P/N 09-38500 "2oz sealant" $17.85). But now that Thiokol's patents on polysulfide sealants have expired seam-sealers that do equally well are available for pennies per pound instead of dollars per ounce. Life Industries is one such source. They make a line of polysulfide sealants for marine applications, including a two-part fuel-proof formulation used to calk fuel tanks & bilges.
With aluminum, the sealant-bonding question - getting the stuff to stick - is a no-brainer. Go to your local Home Depot and buy a quart of JASCO ‘Prep & Primer.' Or buy a quart of ‘AlumaPrep' from Aircraft Spruce. Same stuff, chemically speaking. Of course, the aviation-grade' alumaprep is dramatically more expensive.
Degrease then etch the panels you want the sealant to adhere to for thirty minutes in a solution of ‘Prep & Primer.' (The strength of the solution isn't critical. Anything from 1:1 to 3:1 works fine on clean aluminum. If using Alumaprep, follow their dilution instructions.) ‘Prep & Primer' is a phosphoric acid etchant made specifically for galvanized and aluminum surfaces. Scrub the etched surface with a Scotch-brite pad and neutralize with boiling water. The result will be a matte white finish.
To insure greater integrity of your rivet line, you may wish to use countersunk rivets. The dimple adds depth to the rivet line, making it stiffer without increasing its weight. The 120 degree dies you need for poppers are available from Airparts in Kansas City (www.airpartsinc.com) for about six dollars. And from other folks, too. The dies are used with your regular pop-rivet gun. Of course, if you have a lathe it takes only a few minutes to make such a set of dies and even un-hardened they'll last for several plane's-worth of dimples.
If you've never used flush-head poppers, run a few rows of sets before tackling the tank. The dimpling process enlarges the hole. The geometry here is subtle so be cool, work at the pilot-hole level, opening up the hole to rivet-size after you've dimpled & fitted the row. If you don't, the rivet will be too loose to pop; it'll just pull out. Be very careful when deburring as you'll be working on a corner instead of a flat. A file may be a better choice than a regular deburring tool. (These factors have probably contributed to the Conventional Wisdom that sez flush-head poppers don't work very well. They work just fine, but only when they fit the hole.)
With a pitch of about an inch aluminum poppers provide more than adequate strength for this application. Indeed, the stiffness of steel flush-head poppers dictates a minimum metal-depth of about forty thou. Anything thinner and you're liable to pull the popper through the hole before it can form a large enough shop-head to snap the mandrel.
You can buy flush-head aluminum poppers from J.C.Whitney in boxes of 500 for about $12. (JCW item# 14xx4090A, box of 500, $12.19) The short ones do fine for this type of job.
As with all poppers, be sure to wash them good in MEK prior to use. The manufacture of pop rivets always leaves some amount of lubricant on the finished product. That tiny trace of oil will interfere with the adhesion of the sealant (and of your zinc chromate, when using steel poppers on your other panels).
Steel poppers may be a wiser choice for attaching the flanged aluminum fitting for the tank's outlet. You may of course use steel button-head poppers for the entire tank if you wish. I like the flush-head aluminum jobbies for the seam-lines because they give me a stiffer joint at less weight.
If you prefer to use solid rivets you'll need to provide access for bucking the things. Wag Aero still sells sealed blind-nuts for a reasonable price (Cat# L-676-000 Pkg of 50 for $10.95) Sealed blind nuts are standard for fuel tanks. The threads of the screw are sealed away from the contents of the tank inside a little dome. Slosh the tank, you can still remove the access panel.
Tanks tend to violate the rule for panel size vs edge support so you'll probably want to pound an ‘X' bead into the four sides and the bottom. If the top of the tank is curved it will already be stiff enough but the sides & bottom will tend to be pretty wimpy, especially if you're using soft aluminum. (Almost anything will do for making a tank. Don't tell anyone but I've made tanks out of siding aluminum. )
If you don't understand what I'm talking about here, take a look at a steel Jerry can. Some guys like to roll such flutes into the panel but you can do perfectly well by making up a suitable groove or gap in a board, laying the panel across it and making several light passes along the groove with your rubber mallet.
Fuel tank is usually an irregular box. Occasionally an irregular cylinder. Cylinders support themselves but boxes don't. If the thing has corners, plan on adding a couple of baffles, not only to control the slosh but to stiffen the structure.
Rivet-on flanges for the filler and outlet are available from aircraft suppliers but since they are simple turnings they are easy enough to make if you have a lathe. And even if you don't. There are thousands of hobby-machinists on the Internet, their weapon of choice a little 7x10 lathe that's plenty big enough to whip out a set of fuel tank fittings. To track down such folks just go to the appropriate Newsgroup – rec.crafts.metalworking is but one of dozens of such groups – post a message having ‘Help!' as the subject line, describe the job and tell them where you're located. The squeaking wheel gets the grease - keep shouting until you connect with someone in your area. Like all machinists - which is what these folks are... the size of the machine has nothing to do with it - he'll need an accurately dimensioned drawing to work from and you'll probably need to provide him with the stock. Applying sealants is messy as hell, especially Pro-Seal and the other polysulfides. Masking off the area to be sealed/riveted will help and you might want to consider PK's instead of clecos. Polysulfide sealant is close to the perfect adhesive, it'll stick to anything... and doesn't like to come off. (For dimpled holes you'll need the longer (ie, 3/8") PK's.) Grubby-up a PK, throw it away. You're out maybe two cents.
Give the surface to be sealed a final wipe-down with MEK (or whatever solvent is recommended for you sealant). Allow it to evaporate. Apply the sealant according to the instructions. Most call for a smooth, uniform coating on both surfaces. Not too thick, a few thousandths is all you'll need if your rivet-line is a good fit. And not too wide, about three-quarter of an inch, max. Most of this stuff cures by reaction with water vapor in the air, something present everywhere on our particular planet. Cure time is a function of the width of the sealant-line and the humidity in the air. (That's why the stuff is so popular with boaters – it cures underwater faster than out of it.) The two-part formulation cures faster than the no-mix stuff.
You only need about three ounces of sealant for a ten gallon tank, most of which will go on the flanges, your tools and your clothes. (If you've never used Pro-Seal before, buy a pint :-)
If you use the 2oz kits, the little tubes can be hard to handle without the matching gun, a $75 item. (Aircraft certified, right? :-) A dime's worth of Bondo will allow you to modify a regular calking gun to accept the 2oz aircraft-certified cartridges. If you want to go that route, I'll tell you how to make an adapter.
Aircraft Spruce also lists the stuff in pints [$37] and quarts [$74] but the secret to using bulk-packaged sealants is how to mix & apply the stuff without gluing yourself to the wall. The usual mix ratio is 10:1 and is fairly critical. The use of a ratio'd balance beam, baggies for one component and a Teflon cup for the other is a fairly common procedure. Once you've balanced the beam, pour the One-stuff from the Teflon cup into the Ten-stuff in the plastic baggie then seal up the baggie and mix the stuff by squishing the baggie, like colorizing oleomargarine in days of yore. (That's my yore, not your yore.) Once the color is uniform, snip a corner of the baggie and squeeze the stuff out like decorating a cake, using a scrap of metal as a palette knife to smooth the bead to a uniform thickness across the bond line.
Standard practice when using poppers with a sealed structure is to dip the degreased popper in the sealant just before you stick it in the hole and give it a little twist. Don't get too far ahead with the sticking & twisting before coming back and doing the popping. The structure should be perfectly secured with a PK in about every fourth hole giving you three poppers in a row. When you pop, always do the middle one first. Once it's popped some guys butter a smear of sealant into the mandrel hole but it's not necessary if you're going to slosh the tank.
After your tank is fabricated leave it alone for about three days, until the sealant is cured. After it has cured you can provide yourself with virtual 100% leak-free assurance by sloshing the tank with a PVA fuel tank sealant. J. C. Whitney will sell you a quart of the stuff for about thirty bucks. You need less than a pint but I haven't found anyone who'll sell me that small a quantity. As with the polysulfides, there are only a few companies that make fuel tank sealants and most use functionally identical formulations. The thirty dollar stuff from J.C.Whitney appears to be the same as the hundred dollar stuff with ‘aircraft-certified' on the label. Maybe it's not but it works the same.
Since your tank was already etched, the sloshing sealant is going to form a perfect bond. Plug the outlet, pour in the sloshing sealant (it's a creamy white stuff; the vehicle is MEK) seal up the inlet (I use a hose clamp and piece of inner tube) then commence rolling the tank over and around and up and down... but in a logical fashion. What you want to do is to flow the sloshing sealer over every part of the interior surface.
After sloshing the tank, drain the sealant back into its can and seal it up good. Remove whatever is plugging the drain so air can circulate through the tank, prop it so it can drip out then leave the thing to cure.
Takes about 24 hours.
When it's cured, get your light wand and your bore scope and whatever else you need and inspect the interior surface. It should have a uniform white coating.
The sealant is a form of PVA – polyvinylalcohol. Once cured, it is impervious to virtually all solvents, including gasoline, alcohol and water. I've used the J.C.Whitney stuff on steel, aluminum and fiberglas with excellent results (JCW "Alcohol resistant Gas Tank Sealer" item# 12xx8316Y each $28.99). How well it works depends largely on how well you've prepped the surface. Basic rule is to have it perfectly free of grease, including fingerprints. For aluminum, you need to provide some ‘tooth' to the surface, which is accomplished by the etchant (ie, the Prep & Primer stuff).
Lots of ‘expert' homebuilders damn such tanks with faint praise. Sure it works... but real fuel tanks are always welded, yadayadayada... Sure they are, Mr. Expert. (I invite those experts to join me at FlaBob as we refurbish a P-51... and its riveted, sealed and sloshed fuel tanks.)
Your fuel system should have a strainer in the tank. Smartest one you can get is to MAKE YOUR OWN using a short length of 3003 aluminum tubing, slit about every quarter inch with a hobby saw. The typical hobby saw leaves a kerf about .028" wide, much smaller than the mesh of a cheap finger strainer. Cut enough slots to insure the thing will flow enough fuel.
The tank should be equipped with a shut-off valve. See the Northern Hydraulics catalog. Go to the same source for your gascolator. It ain't aviation-certified but it works and doesn't cost the earth.
Basic fuel tank plumbing is to keep a constant down-ward flow of fairly large diameter tubing from the tank to the gascolator. The idea here is that anything large enough to get through the strainer in the tank will not block the fuel line but will simply end up in the gascolator.
If you're flying a VW, a primer makes for easier starting. Since you hand-prop real engines, put the primer near the gascolator; no need to put any more fuel in the fuselage than absolutely necessary. Great Plains sells a good primer at a fair price.
For lo-buck builders, riveted, sealed and sloshed fuel tanks are a practical alternative to other methods of fabrication, their use so common we tend to forget others may not have heard of them.
-R.S.Hoover
PS - The ‘xx' in the JCW item numbers is different for each catalog but the basic number stays the same.
(Ed.Note: Originally posted to the internet in Oct. 2003)
Tuesday, December 5, 2006
VW - The Coca-Cola Myth
> I still keep getting guys telling me how clean n shiny my rusty bolts would > end if I put them for a night on coke
---------------------------------------------------
Bright & shiny? Myth.
But rust-free? True. ( Okay, sorta true.)
Coca-cola (and most other soft drinks) contains phosphoric acid. After eating the enamel off your teeth it gives you the extra zing American's expect in their soft drinks. Coca-cola also contains carbonic acid, created whenever carbon dioxide is dissolved in water. Both are effective de-rusting agents. But not very fast. While Coca-cola is acidic, and while the acids it contains are specific 'getters' of oxides, they are quite dilute; as rust removers they don't work very fast. Raising the temperature and leaving the part in the solution for several days is usually required If you want to remove the rust. But if you simply want to break the bond on a rusted nut & bolt, an overnight soak should do it.
The real question is why someone would use Coca-cola when they are literally surrounded by more effective -- and less expensive -- rust-removing agents. You can buy various acids, including phosphoric, from Home Depot... and probably already have a few around your house. (Tile cleaners often use hydrochloric acid and every old car battery contains some sulphuric acid.) And if all you want to do is loosen a rusty fastener then you should be using Kroil or Mouse Milk. (Yes, 'Mouse Milk.' It's a brand name. See any good machinist-supply catalog.)
Got rusty bolts? The least expensive method of restoring them is simple reverse electrolysis. That is, a conductive solution (salty water will work) plus a battery charger. Look it up. Several sites on the internet devoted to cleaning metal.
(It might also be a good idea to read what's in the stuff you eat & drink. The world is full of surprises :-)
---------------------------------------------------------------
Ed. Note: The above got several people to try electrolytic rust removal for the first time, with results that ranged from delightful to mushroom clouds and prompted the following:
----------------------------------------------------------------
Electrolysis -- You gotta clean the thing first.
Yeah, I know... it's a mess. But the odds are it's an oily, greasy mess. And there may still be some paint under all that rust. Electrolysis does not work on grease. Nor paint. You'll end up removing the rust from all around the greasy or painted part... which may be what you want but usually isn't. So degrease it.
A hot solution of lye (ie, the traditional 'hot tank') is the time-proven method. Of course, if you get some on you, you tend to jump around and make funny noises. TSP -- trisodiumphosphate -- is a more benign getter of grease and does a pretty good job on paint. Just keep boiling the dirty part until it's down to Basic Rust then pop it into your electrolytic bath. (Be sure to use real tri-sodium phosphate. There is a common household cleaner with the BRAND NAME of 'TSP' that does not contain any phosphates at all. Paint department usually carries the good stuff [you use it to scrub old paint before laying on new]. )
Same holds true for your iron electrodes. If you clean them before wiring them up, they will have more effective surface area.
Handiest clamps I've found were pieces of copper pipe. Cut off a piece about an inch long, wrap your wire around it and solder, then drill the thing to accept at least three sheet-metal screws. Slide over the re-bar, tighten the screws, connect the wire and away you go. The re-bar gets eaten up but the clamps will last just about forever.
If you've used a concentrated salt solution for your electrolyte then you'll need to BOIL the part in clean water once the rust has been removed. The derusted, boiled part will develop a haze of rust as soon as you lift it from the boiling water so be ready to deal with it. Either give it a shot of primer as soon as it's dry or hose it down with WD-40. (I don't recommend the use of lye as a electrolyte. Any salt (as opposed to acid) will work. I use washing soda.)
A big advantage to electrolytic rust removal is that it only takes away the rust, not the metal attached to it. With sand blasting, everything goes -- and leaves a surface that's rough as a cob.
Save the Coca-cola for rotting out your teeth the way God and the American Dental Association intended. If you got rust, there are smarter, less expensive ways to get rid of it.
-Bob Hoover
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Bright & shiny? Myth.
But rust-free? True. ( Okay, sorta true.)
Coca-cola (and most other soft drinks) contains phosphoric acid. After eating the enamel off your teeth it gives you the extra zing American's expect in their soft drinks. Coca-cola also contains carbonic acid, created whenever carbon dioxide is dissolved in water. Both are effective de-rusting agents. But not very fast. While Coca-cola is acidic, and while the acids it contains are specific 'getters' of oxides, they are quite dilute; as rust removers they don't work very fast. Raising the temperature and leaving the part in the solution for several days is usually required If you want to remove the rust. But if you simply want to break the bond on a rusted nut & bolt, an overnight soak should do it.
The real question is why someone would use Coca-cola when they are literally surrounded by more effective -- and less expensive -- rust-removing agents. You can buy various acids, including phosphoric, from Home Depot... and probably already have a few around your house. (Tile cleaners often use hydrochloric acid and every old car battery contains some sulphuric acid.) And if all you want to do is loosen a rusty fastener then you should be using Kroil or Mouse Milk. (Yes, 'Mouse Milk.' It's a brand name. See any good machinist-supply catalog.)
Got rusty bolts? The least expensive method of restoring them is simple reverse electrolysis. That is, a conductive solution (salty water will work) plus a battery charger. Look it up. Several sites on the internet devoted to cleaning metal.
(It might also be a good idea to read what's in the stuff you eat & drink. The world is full of surprises :-)
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Ed. Note: The above got several people to try electrolytic rust removal for the first time, with results that ranged from delightful to mushroom clouds and prompted the following:
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Electrolysis -- You gotta clean the thing first.
Yeah, I know... it's a mess. But the odds are it's an oily, greasy mess. And there may still be some paint under all that rust. Electrolysis does not work on grease. Nor paint. You'll end up removing the rust from all around the greasy or painted part... which may be what you want but usually isn't. So degrease it.
A hot solution of lye (ie, the traditional 'hot tank') is the time-proven method. Of course, if you get some on you, you tend to jump around and make funny noises. TSP -- trisodiumphosphate -- is a more benign getter of grease and does a pretty good job on paint. Just keep boiling the dirty part until it's down to Basic Rust then pop it into your electrolytic bath. (Be sure to use real tri-sodium phosphate. There is a common household cleaner with the BRAND NAME of 'TSP' that does not contain any phosphates at all. Paint department usually carries the good stuff [you use it to scrub old paint before laying on new]. )
Same holds true for your iron electrodes. If you clean them before wiring them up, they will have more effective surface area.
Handiest clamps I've found were pieces of copper pipe. Cut off a piece about an inch long, wrap your wire around it and solder, then drill the thing to accept at least three sheet-metal screws. Slide over the re-bar, tighten the screws, connect the wire and away you go. The re-bar gets eaten up but the clamps will last just about forever.
If you've used a concentrated salt solution for your electrolyte then you'll need to BOIL the part in clean water once the rust has been removed. The derusted, boiled part will develop a haze of rust as soon as you lift it from the boiling water so be ready to deal with it. Either give it a shot of primer as soon as it's dry or hose it down with WD-40. (I don't recommend the use of lye as a electrolyte. Any salt (as opposed to acid) will work. I use washing soda.)
A big advantage to electrolytic rust removal is that it only takes away the rust, not the metal attached to it. With sand blasting, everything goes -- and leaves a surface that's rough as a cob.
Save the Coca-cola for rotting out your teeth the way God and the American Dental Association intended. If you got rust, there are smarter, less expensive ways to get rid of it.
-Bob Hoover
VW - Hoover on Trikes
> I've seen trikes that run without a fan or engine tin at all, and >that's one option I'm considering.
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Don't. Unless you live in Norway.
The cooling equation calls for a given rate of air flow down through the fins. Keep that in mind all the time because no matter what it looks like, the VW is not an airplane engine; it wasn't designed to use ram-air cooling. It uses an engine-driven impeller to pump air into plenum chambers where it is directed to the corners of the engine (ie, the hottest parts) and then forced down through the fins which are drafted according, having an outlet area slightly larger than the inlet to accommodate the expansion of the air as it absorbs heat.
What you got to get a handle on is that to make the flow go, there has to be difference in pressure. It's not a lot and is normally measured in inches of water instead of inches of mercury and mercury is heavier than lead for crysakes... about 14x heavier than water. All of which means you're not talking about a lot of pressure and that means it is easy to fool yourself that things are getting lotsa air when they ain't.
How much? Airplane engine, designed to use ram air, with forged heads and machined fins on the barrels and lots of other neat little tricks, you need a minimum of about 6" of water pressure-differential equivalent to keep the engine's temps within spec.
VW, with its cast heads and fins needs more. A lot more. Such as 9" of water.
What's that in miles per hour for ram air? 120, 140, something like that. Doesn't really matter because whatever it is, sure as babies shit green you ain't going to see it on three wheels with a VW under your butt.
Standard Day is pretty cool. Run up to visit the Presidents, you won't see many Standard Days coming or going. Sturgis isn't Narvik and that kewl Norsky bike we all admired in the magazine, with its chromed barrels and chromed valve covers and no shrouding and all that other neet stuff has probably got a trailer & a Volvo to pull it tucked into the trees just out of the photo.
It really gets down to if you want to talk three-wheeling or ride them. If you want to ride, you need a reliable engine. You can hang all the chrome on it you want but you can't fool with Mother Nature and she's the gal that wrote the book on thermodynamics. So dress it up. Blind those fools! But under the shouding, inside the engine, you gotta obey the rules because it ain't nice to F**k with Mother Nature.
If you go with one of those Taiwanese blower housings... ( "Early Porsche Style!" All bullshit, of course. The round Porsche housing weighed about as much as Buick; that piece of Taiwanese crap will fall apart if you yell too loud.) ...you'll have to tinker with it to get it accept the thermostatically controlled air vanes, then you'll have to make a connecting rod to fit. The key point on the air vanes (yes, you need the thermostat and all the junk that goes with it) is that they have to align with the middle fin on the heads when fully open. The air vanes have their own frame. Look at a regular blower housing to get some idea where the holes have to go then put the thing in position and start tinkering. (Yes, you need the engine assembled and the alternator mounted).
Paint the crankcase with a light coat of flat black Rustoleum. Ditto for the valve covers and your push-rod tubes. Yeah, I know -- everybody else sez.... Just look them right in the eye and say, 'God told me to do it,' then shut up and keep looking at them. You can repeat it up to three times if you have to. After that it's obvious you're dealing with a heathen so just go ahead and deck the sucker. (Hanky and a roll of quarters; dimes if you got small hands. Wind the hanky tight. Bust a knuckle, you can't shift for shit.)
Balance everything to a gnat's ass or finer. Lighten up your flywheel. You won't be pulling any stumps so go ahead and pick a wiggle stick that'll let that puppy rev. You don't need a lot of cubes for a trike. Machine-in 88's are smart; even stock works good. Run an oil filter but not a filter/pump adapter unless you got one that actually fits; do it right. Round up the parts to convert your crankcase to use a dog-house cooler. You can run one of those itty-bitty Porsche-type blower housings if you want, just make sure you get one that will accept the thicker fan and that has all the bits & pieces for the dog-house. And you gotta run all the tin-ware. Use cool-tin under the jugs but you got to modify the lower shield to accept the cool-tin plus you gotta modify the lower shield to mate with your J-tubes.
What you're creating here is the lower plenum chamber of the cooling system. It has to be sealed off from any ram air produced by your forward motion but open to the rear, where you want a lip to create a good suction zone. You got to do all this and get it to fit perfect -- no gaps or leaks at all -- then you got to spend about a thousand hours making it smoother than snot on a door knob before you take it to the plater and see if he'll accept it. You know the guys I mean; the ones who still know how to do real chrome, starting out with a layer of copper then a layer of nickel then a layer of chrome thick enough to shrug off salt water, cigar butts and the occasional state bird of Texas.
Now, you see that dog-house oil cooler? See that cute little exhaust duct, where it's supposed to poke through the forward breast tin that you ain't got? For homework this weekend I want you to whip up a duct that arches down and under the lower shrouding. Doesn't have to extend aft very far, just so's its below the shrouding and pointing south when you're going north. Bell the mouth a bit to provide some suction. Put a couple of nut-serts on the lower shrouding to support it. You'll probably have to safety-wire the bolts.
If you promise to never go over 5000 rpm, not smoke, don't use any swear words and be home by midnight you can use a stock crankshaft. Otherwise you want one with counterweights on the flanges and you want it and the flywheel and the pressure plate and the fan pulley all assembled and torqued and balanced as a unit.
Don't look at me like that, it's not my idea it's Mother You-Know-Who with her idiot son Isaac Whats-his-name and all those Laws of Motion bullshit, with his phlugoid motion this and axis of rotation that.
Yeah, I know. None of your buds do it that way. But when you put the hammer down most of your bud's engines start doing the River Dance. Yourz'll be doing the lambada.
I guess you know you can't fly a trike. You've been on one, haven't you? You know you can't use your elbows as curb feelers anymore dontcha? All you can do is sit there and steer. Like a geezer on his Courtesy Cart at Wal-mart, right down to the cute little flag on the antenna. Trikes ain't bikes and you gotta learn to live with that. (Find an old 8-track, put it in there somewhere. Couple of Lawrence Welk tapes where guys can seeum. Mention how much you and 'the little woman' enjoy your 'motorcycle.' Shuts them right up, except for the snickers. But when you blow past them doing ninety going up a grade they won't know whether to shit or go blind :-)
So whatllya get for all that horsepower-wise? Not as much as all the bullshiters claim but more than you'll need to kill yourself. You'll probably end up with squirrely dual-carb arrangement that'll wear out the engine from running rich and sucking in dirt through those tea-cozy air filters before you ever get the thing running right. But anything other than a dualies, you'll have trouble with carb and manifold icing because of the exhaust system you'll probably use. Nothing wrong with dual carbs -- VW used them on the Type III & IV, as did Corvair and lotsa others. But most of the after-market dual carb kits are crap. Unless you spend some serious bucks for the Good Stuff, the cross-bar will fall apart the first time you rev the engine, the filters are a joke and those push-on fuel hose fittings are as scary as an ex-girlfriend waving a MAC-10.
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Despite all that, get it running, it's a lot of fun, assuming you remembered to do give it a brake job :-)
Now, what all the above was in lieu of is the fact there's a world of difference between something that runs and something that runs sweet. Starts at the first touch of the button. Nice idle. Twist it and she winds right up, no stumbling or hesitation or jerking around. And it not only runs sweet, it runs cool. After your dog, death and taxes, a properly built engine is the one thing in your life you can count on. I'm not just talking reliability or durability, I'm talking old fashioned loyalty. You can't buy it and you can't bolt it on, you have to build it in.
-Bob Hoover
PS -- the gear shift linkage doesn't have to be such a pain in the ass. To support the gear shift lever, try and get ahold of the shifter out of a bus. It's got a supporting sleeve on the front that pokes through a bushing in the shifter frame. You're going to be higher up and at an angle to one side or the other, as comapred to the shift-rod installation in a bug or bus. Don't try to over-engineer the thing, just go ahead and weld a stub onto an old coupler so the stub sticks up above the torsion bar housing where you can get at it. Use aircraft style ball-end fittings. You want at least one threaded section so you can take out the slack as wear accumulates. The shifter-rod don't have to be straight, it'll still work even with some bend to it.
Good luck with your trike and write if you find work :-)
Monday, December 4, 2006
CC'ing Your Heads
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(From the unpublished manuscript 'How to Build A Reliable Aircraft Engine')
Most folks arrive at this point not because they are building an engine but because they are repairing one. Doing a valve job on a VW engine alters the chamber volume. Or they've dropped a valve and have had to replace one of the heads. Replacement or repair of a head alters the chamber volume and chamber volume is a factor in the compression-ratio equation. Before they can re-assemble the engine they have to measure the chamber volume and re-compute the CR to ensure the repair work has not upset the compression ratio.
Measuring chamber volume is easy to do and the procedure is all the real service manuals for the VW engine. But just to keep all the information in one place, I'll give you a quick sketch of the procedure.
Using the catheter-type syringe and a sealing disk similar to those shown in the photos, you should be able to measure the volume of a Volkswagen combustion chamber to within 2cc. That is, if you measure it a multiple number of times your results should not vary from the average result by more than plus or minus 1cc. (As a general rule, always measure each chamber at least three times.)
Herez how: Install a spark plug in the head and torque to spec. If the valves are not perfectly fluid-tight, pop them out and put a light smear of Vaseline on their sealing surfaces. Level the head in both directions then smear a light coat of Vaseline around the periphery of the sealing disk and drop it into the chamber. If the sealing disk is 3/8" or thicker it will be heavy enough to stay in place by itself (the specific gravity of cast acrylic sheet is only 1.19). For thinner sealing disks you'll need to hold them in place with your thumb while filling the chamber or add a few ounces of weight to their edges.
The disk(s) doesn't have to be perfectly round. You can saw the thing out with a jig saw, smooth the edge with a file and you've kept it to within +/- sixty thou or so, it will work perfectly well. Nor do you need five holes in the disk as shown in the factory workshop manual; you can do a quick check with just one.
Using a suitable fluid, fill the chamber and the hole through the sealing disk. Subtract the volume of the hole from the total. Do both chambers. For OHV engines I think you'll find ‘wetter' water does a better job than the traditional kerosene. This is because you need to measure the chamber several times and average the result and that means you have to remove all of the fluid from the chamber. With a flat-head engine it's pretty easy to simply wipe the chamber dry but with an OHV the kerosene tends to cling to all the nooks & crannies. So use water with a drop of wetting agent as your fluid. When it comes time to dry the chamber, simply dump it out, slosh it with a bit of alcohol then blow it dry. Alcohol is hygroscopic - - it will mix with any remaining water and once mixed with alcohol it takes only a modest blast of air to dry even the deep recess around the spark plug's insulator.
A quick check using the syringe shown in the photos will allow you to determine the volume of your combustion chambers to within 2cc (i.e., plus or minus 1cc). To compute your compression ratio use the largest volume you measured as the factor in the CR equation.
That's it. If you're a shade-tree mechanic you're all done. Good luck in the contest.
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CC'ing TOOLS & EQUIPMENT
Why am I doing this?
Truth is, some of you aren't. Although this article was written to explain how to improve your engine's efficiency by adjusting it's volumetric balance, many builders are interested only in determining their compression ratio. If that's all you're here for, drop down to MEASURING CHAMBER VOLUME.
By applying modern-day standards of balancing, of both mass and volume, you can pick up as many as ten free horsepower from a bone stock VW engine. And I mean ‘free' in the sense that it will use the same amount of gas. That seeming impossibility reflects the fact the engine must first overcome any internal imbalance before it can deliver any usable power. That means an imbalance generating a loss of five horsepower costs an additional five horsepower to balance the books, resulting in a net loss of ten horsepower. This is a key factor in the importance of balancing and one most people overlook. That is, for each imbalance you eliminate you will gain approximately twice that amount of improvement.
Of even more importance to builders of flying Volkswagens is the fact that a properly balanced engine is more durable and the reduction in wear is even more pronounced than the improvement in fuel efficiency. That's because most of the internal imbalances appeared in the form of friction and heat, which could only be overcome by generating more friction and heat. The bottom line is that a 3hp imbalance could cost you up to four times that in additional wear. Eliminate the imbalances and the engine's wear-factor takes a dramatic drop.
The benefits of mass-balancing became self-evident as the normal operating speed of internal combustion engines was increased but the role of volumetric balance was not fully appreciated until computers came along. Once they had a super-computer to play with automotive engineers discovered that a lot of what we thought we knew about the process of combustion was not entirely correct, that there were transient phenomenons due to volumetric imbalances that we'd attributed to other causes. This is one of those non-intuitive kinds of things, not easily explained without a bit of background but the basic reason has to do with the process of combustion and the fact those seemingly insignificant differences in volume appear on both sides of the compression ratio equation. Turns out, relatively small variations in volumetric balance can produce some relatively large losses of power. Since that time they've taken exquisite pains to reduce those imbalances. How this is accomplished in a mass-production environment is quite interesting (at least, to me :-) but the bottom line is that we can take advantage of what they've learned by simply paying more attention to the volumetric balance of the engines we build.
Thanks to computerized equipment a modern balance shop can do mass-balancing to a fraction of a gram at a very reasonable price. But matching the volume of a pair of heads (i.e., four chambers) to a tenth of a cubic centimeter remains more art than science. In the racing world it isn't uncommon to pay a thousand dollars for a pair of full-trick heads, their volumes accurate to 0.1cc across all four chambers. The cost isn't in the tooling; it doesn't take a lot of tools for head-work. The money goes to buy the head-man's time. That fact is in our favor because time is about the only resource of which lo-buck homebuilders have a surplus. By acquiring a few tools and devoting a bit of time to your heads, you can bootstrap yourself into a better engine than you can afford to buy.
At this point a majority of non-mechanic home-builders are sitting there with a large black question-mark floating in the air over their heads. If adjusting chamber volume is more art than science, how can they hope to tackle the job themselves?
The answer is so simple it will make you smile. Anything you do to reduce the magnitude of the existing imbalance will improve the efficiency and durability of your engine. The fact the pro's regularly balance to a tenth of a cubic centimeter is like running the four minute mile. But we're just a bunch of week-end joggers so let's not even think about that degree of precision. At least, for now. Instead, let's look at the basic problem.
You have four chambers. You measure them (the procedure is outlined below) and come up with four values, probably accurate to within half a cc or so. Using real numbers now... two heads measured 56.5, 58, 58, and 60cc. Rather than trying to turn you into a head-flow guru capable of achieving 0.1cc accuracy on a repeatable basis the question becomes: "Can we reduce that 3.5cc spread to something smaller?"
The answer is a decided ‘Yes!' And it really isn't all that difficult because you only have to work on three of the chambers. That is, you can't make a chamber any smaller so the largest chamber becomes your target-size. So relax. You can do this. And you'll end up with a better engine because of it.
Then comes the question, how much metal are we actually talking about? Just what the hell is a ‘cubic centimeter'? The answer to both: Not much.
If you've been following the Practice Wing project you should have some high-density urethane foam on hand. Print out the ONE_CC drawing, glue it to a piece of foam then use a razor blade to make a cube the size of the printed square. That is, a cube 0.3937" on a side. No foam? Then make the cube out of wood. Or plastic. Or tool steel (if you can). The goal is to give you a tangible reference to the volume of a cubic centimeter.
Hold a cubic centimeter in your hand and three things are immediately apparent: Firstly, it isn't very large. Just a tad over 3/8" on a side. Secondly, you can appreciate the heroics it takes to achieve repeatable tenth-cc accuracy. And finally, you can see why a lot of people consider such imbalances to be insignificant. To the uninitiated, something that small has to be unimportant; something they can ignore.
Got some sixteenth-inch ply? Okay, howz about some poster board a sixteenth of an inch thick. Or a piece of .063 aluminum. Three-eighths wide, six inches long, you're looking at about one cc. Now go back and look at what we're trying to do. To balance the 56.5cc chamber we need to remove three and a half cc's of material. Just unshrouding the valves is usually good for that volume.
So how do we do all that? I'll get into that in a minute but it all begins by measuring the volume of your combustion chambers.
MEASURING CHAMBER VOLUME
If you've never cc'd your chambers, relax; it isn't difficult nor do you need a lot of equipment.
My first exposure to cc'ing heads was in the mid-1950's. I bolted a piece of plex to the freshly milled head of a Ford V-8, leveled the thing up with wedges and used a turkey baster to fill each chamber with kerosene. After doing both heads I plugged the smallest volume into the compression-ratio equation and came up with a CR of about 11:1, assuming I used a sixty-thou gasket. Life was good, assuming I could swipe enough avgas for a couple of runs through the lights :-)
Determining the volume of the combustion chambers is a chore common to all engines and the basic procedure is the same for all. The procedure requires:
1. Some means of holding the head level.
2. Some means of sealing the space to be measured.
3. A method of accurately measuring the amount of fluid needed to fill the chamber.
Working with nothing more than a 1/4" drill motor and a couple of carbon steel rotary files, cleaning up the marks with a strip of sandpaper spiraled around the split end of a quarter-inch dowel, you can expect to produce a cc-job accurate to about 0.5cc across all four heads. Indeed, the usual problem when a novice does his first set of heads is going too far, opening up the smaller jugs to a volume larger than the target-jug. Which means they have to take a bit out of the big chamber, which usually leads the them chasing the volume back & forth until they get sick of it.
SAME PRINCIPLE, DIFFERENT PROCEDURE
The secret to hitting your chamber volume dead-on is exactly the same as for hand-cutting a piece of aluminum to a precise dimension in that you don't shoot for the exact dimension. You always give yourself a clean-up allowance. Cutting aluminum by hand you snip or saw the thing about forty-thou over-size then dress the edge with a file. You do much the same when cc'ing your chambers in that you use the rotary files to get close to the finished size then begin smoothing things up with abrasives, checking the volume periodically.
I mention this now because the next subject has to do with the precision of your cc'ing job and the fact balancing your chambers to a fraction of a cc might take you a couple of weeks (!) if you've never done it before, whereas balancing them to 1cc (across all four) should take only a few hours. If you don't want to spend the time then there's no need to spend the money for an accurate liquid dispenser, such as the one shown in the photos, which is graduated to 0.1cc. Turkey baster or the big syringe, you can work down to 1.0cc with fair accuracy. And there's a big difference in price between a surplus syringe and a certified-accurate, lab-grade burette.
All engines get their chambers cc'd, not just Volkswagens. The cc'ing of combustion chambers is illustrated and talked about in the various manuals on engine building. The three key elements mentioned above reflect the basic core knowledge for any cc'ing job. Then comes a host of details; the How-To stuff, most of which is engine-specific but some of the common factors address the accuracy of the finished job, the degree of precision, how long it takes and various convenience factors. As with most other aspects of proper engine fabrication, cc'ing your heads calls for a keen attention to detail. Most of the time.
The point is that you really don't need much to do a quick check.
Lots of after-market retailers will sell you a ‘cc kit.' It usually consists of a plastic syringe like the one shown in the pictures plus a plastic disk with a hole in the middle.
What's it cost? Hard to believe but some outfits want as much as thirty bucks for a cc kit.
See the syringe in the photos? It comes from a veterinary supply house. New, quantity one, you're looking at three or four dollars. But syringes have a certain shelf life and once they're past it, the supply house often flogs the stuff off as surplus, out-dated or whatever. Buy them a dozen at a time, the syringe shown cost about half a dollar.
The plastic sealing disk is any sort of flat plastic, eighth of an inch thick or more. Doesn't have to be clear. The sealing disks I use for 77mm jugs are made from a hunk of red Plexiglas I dug out of the scrap box at a plastics retailer. In a pinch I've even cast my own using polyester resin on a sheet of waxed glass. It only has to be flat on one side and just clear enough so you can chase the bubbles. Chasing the bubbles is one of those little details no one mentions :-) The bitter truth is that some bubbles don't like to be chased and you can spend the best years of your life trying to coax the thing over to the hole.
The sealing disk in the typical kit has just a single hole in the middle. Cheap, easy to make and so forth. But harder to use than one with more holes. More holes, you don't have to chase the bubbles so far but more importantly, with more holes there's a better chance you'll be working with a truly level head. And that's worth mentioning because a lot of after-market heads, especially rebuilts, the sealing surface is not parallel to the valve gallery gasket rail, which is what you use to sit the head on.
So you sit the head on the gasket rail (after taken the studs & valve stems into account) and use your machinist's level or framing level or whatever on the chamber-side of the head to use wedges and so forth to get everything perfectly level, end-to-end and fore & aft.
That's no guarantee the sealing surface of the chamber is level. But if your sealing disk has a pattern of holes as shown in the drawing (and in the VW factory workshop manual), as the chamber fills the level of the fluid in the various holes tells you if the chamber is level.
You subtract the volume of the holes, by the way. Figure out their volume ahead of time, scribe it right onto the sealing disk. You can use the same equation as for V1, just substitute hole diameter for bore and the thickness of the plastic for stroke. Then multiply that by however many holes you have and be sure to always fill the hole to precisely the same level.
Buy a cc kit, it usually comes with only one disk for each size of cylinder. Indeed, some outfits only sell you one disk, period, screwing you to the cash register if you want two disks or a set of a different size. Which is another example of how VW after-market retailers screw the kiddies, because to do the job properly you really should do both chambers at the same time... and that means two disks of each size.
Making disks is pretty simple, assuming you understand plastic. Drills for use with plastic looks like a needle :-) We usta call them ‘canopy bits' or ‘Plex bits.' Included angle of something like 30 degrees. If you don't have a set of bits for plastic and don't know how to sharpen a drill bit, you'll have to use a hand-drill. Even then, you're liable to crack it when the bit breaks through the other side. That's when it grabs the flutes. Plastic bit, you needle a pilot hole then flip it over, finish it from the other side. Slow speed, always. Kerosene makes a good lubricant for thicker stock.
Although the sealing disk drawing I've posted shows dimensions to thousandths of an inch that's just the math. You can cut one out with a jig saw and it'll work just fine. That's because the sealing lip inside the combustion chamber is at least a tenth of an inch wide. So long as the disk is close to a true circle and makes good contact with that lip, it will work okay. (Too big? Then sand the edge.)
Once you understand the purpose of the sealing disk you'll see them all around you, just waiting to be cut out. See that translucent plastic over the waterproof light in the bathroom? That'll work. You can still see the bubble. Got some curvy scraps from an a blown canopy? That'll work. Just put it on a heavy plate of polished aluminum, pop it in the oven set at about 300 and let it flow out flat.
Want to keep your sealing disks nice & pretty? Got some old 5-1/4" floppy disks? Cut one open, throw away the disk, store your sealing disks in the sleeve.
To make your chamber water-tight you smear a little Vaseline on the valve seats and a light wipe around the outer-most edge of the plastic disk. Then you fill the syringe to a precise level and use it to fill the chamber. Subtract what's left in the syringe from whatever you started with, then subtract the volume of the hole(s) in the plate and that's the volume of your combustion chamber, accurate to however accurate you are and as precise as the divisions on your syringe.
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On the other hand... there's lots of lab equipment scaled to 0.1cc increments but most of it lacks the volume to fill a chamber. VW combustion chamber volume may be as small as 40cc or as large as 80cc, depending the size of your valves and the displacement of the engine. A burette large enough to hold one chamber's-worth of juice and accurately marked to 0.1cc precision, you're talking about an expensive piece of goods. The high price of the Good Stuff comes as a shock to guys building just a single engine, often causing them to settle for half a loaf, using a turkey baster of whatever.
So use a marble or two. And a smaller but more precisely marked burette.
Got a marble? There's a free one in every can of rattle-can paint. (That's the rattle :-) Or you can buy a bag of them at the dime store. Or use steel ball bearings (but remember they rust). I like ball bearings because of their uniformity. Marbles vary by quite a bit.
So what's the volume of your marble? Half fill your burette, bring the meniscus to the line, make a note, then drop your marble into the burette. Make another note then do the math. (You may shout ‘Eureka!' if you wish :-)
By adding objects of known volume to your combustion chamber you can use a smaller dispenser marked to 0.1cc graduations. Just be sure to keep the numbers straight. (KEEP GOOD NOTES!).
When trying to achieve the smallest possible difference across all four jugs, statistics are your friend. Make all of your measurements a number of times and then average them. This helps to reduce the human error, a factor that can be deadly for the first-timer. Without experience you can't appreciate the significance of seemingly minor differences. Do everything several times, throw out the high and the low then averaging the remainder tends to reduce any errors of observation.
Work with good light. Don't be shy about using a reading glass to inspect the meniscus in the sealing plate holes and your burette. When taking a series of measures on the same chamber(s) always be sure to start with it perfectly dry. That means you'll need an air blast to blow out the spark plug (which should be torqued to spec on a new washer). Before filling the chambers I swab them with lacquer thinner or MEK to remove any oily residue that might prevent the fluid from fully wetting the surface. When applying the Vaseline, don't get sloppy -- a little dab will do ya.
Once the sealing disk is in place, it don't wanna leave :-) Use a dental tool or bend a tiny hook on the end of a piece of wire to lift it out using one of the holes. Pour the water out of the chambers, wipe them down with a clean towel, give it a slosh of alcohol and blow the plug dry.
Traditionally, the liquid used for cc'ing heads was kerosene. (I donno... it just was.) Working with VW's I've heard guys advocate the use of everything from anti-freeze to ATF and big dummy that I am, I tried them all.
Hell of a mess.
Light oil of any kind is a contaminant in the shop and can be a fire hazard. And glycol, Prestone or what-have-you is poison. In the mid-70's I started using ‘wetter' water for cc'ing and found it gave more consistent results and a lot less mess. How do you make water wetter? Originally, I added a couple drops of ‘Photo Flow,' darkroom stuff that prevents water spots on your negatives. But for cc'ing heads, a couple of drops of liquid detergent per gallon of water has about the same effect. A drop or two of food coloring will make it easier to see in the burette. Try to work in a room-temperature environment. The temperature of your measuring fluid and the heads should always be the same. (Keep the liquid in the shop with the heads. You'll need to do rough spot-checks as the works progresses. I'll have more to say about that in a minute.)
I normally use a burette similar to the one in the photos. You'll notice that it's attached to a standard that bolts to the base, which is a plywood box filled with concrete. To attach the burette to the standard, fancy name for a scrap of plywood, I twisted some welding rod around a bolt a little smaller in diameter than the burette so the resulting hair-pin-looking piece would grip the burette. To fasten it to the standard I drilled a pair of holes, poked the legs of the hair pins through the holes, bent them over and covered the bent ends with a slather of Bondo. (You can see all this in the pictures.)
The base is heavy to prevent it from tipping over and breaking the burette. It is leveled with shims or screws then the burette is over-filled using a pitcher and funnel. To dispense the liquid I use a length of surgical rubber tubing (drug stores carry it) fitted with the glass tip from an eye-dropper (American Science & Surplus (www.sciplus.com) sells all this stuff, including a pretty good Chinese burette). Flow is controlled by a clamp on the rubber hose. Once the burette is over-filled I bring the fluid down to the line by releasing the clamp and returning the fluid to the pitcher or bucket.
To fill the chamber I hold the eye-dropper/nozzle over one of the holes then play with the flow-control clamp until I bring the fluid level to the base of the sealing disk then chase out any bubbles by tapping on the disk with my fingernail or the handle of a dental tool. Once all the bubbles are accounted for I fill the central hole a drop at a time until all of the holes are at precisely the same. Then I read the burette, hunkering up or down to eliminate any parallax.
Measuring a chamber takes me less time than it took you to read the paragraph above. Using two disks allows me to do both chambers with one set-up.
Then I clean everything up and do it all over again.
To determine the finished volume, as when I'm down to the point of making fractional cc adjustments, I measure each chamber as many times as I think I have to. Sometimes I'll get three identical measurements in a row on both chambers. And sometimes I'll do it half a dozen times and get a different measurement each time, usually when I'm tired or whatever. (People wonder why I don't answer the phone or throw rocks at them when they arrive unannounced. Now you know :-)
That's how I do it when I'm balancing the volume of the heads, trying to achieve 1cc or better across all four. When I'm in the shop working on the heads, I generally use a syringe. It's only accurate to 2cc but it's a lot faster and when you are first opening up the heads, 2cc accuracy is more than enough.
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When building a big-bore engine, cc'ing your heads begins long before you get around to calculating your compression ratio. In fact, it starts when you first acquire the heads. If they're new, with valves already installed, just install a spark plug, grease them up and do a rough measure of volume. With bare heads or heads that have just been overhauled (ie, welded-up with new seats, etc.), rough in the valves then install a plug. Level them up, select the appropriate sealing disk (if you haven't opened them up they'll be some other size) and use a baster or syringe to get some idea of their as-cast volume. It's handy to write that volume directly onto the head using a crayon or wax pencil. The heads should already have been documented and assigned a work number and a documentation package (sounds complicated; actually, each head is just a page in a notebook.) A lot of folks think that's overkill for just one engine but during the course of the work you must have some means of identifying the chambers and even one engine has four of those suckers in two identical heads. If you're building a really good engine, you'll make up a set of spare heads, identical to the first set. Having a spare set of heads on-hand allows you to swap heads when necessary and do the valve work at your convenience.
When you open up your heads for bigger jugs you are trying compress the swept volume of the larger jug into the original compression space. There's no doubt the compression ratio will go up. The big question, is how much?
If the only thing that changed was the swept volume and the deck clearance was kept the same, going from 1600cc to 1834 is going to raise your CR by about one point. But that's the myth that leads to blown engines because nothing remains unchanged. When you open up the heads even the slightest skim cut will reduce the chamber volume. Another part of the myth is that most guys don't know what their original compression ratio was to begin with. Later model engines ran 7.5 or 7.7 (the Export crate engine from the Puebla plant is 7.7) depending on the heads but if it's been overhauled a few times you could be using heads that have been flycut. Bottom line: It isn't uncommon to see 1834's with a CR of 9:1 or more.
Opening up your heads for bigger jugs offers the opportunity to unshround the valves. Unshrouding the valves results in a profound increase in flow-rate for the same amount of lift. The procedure is covered in Bill Fisher's "How to Hot-rod Volkswagen Engines" but I'll try to insert a couple of photos to show you what I'm talking about.
To unshroud your valves you have to remove metal from the chamber. Any time you remove metal from the chamber you need to cc the chambers to make sure they all end up the same size. This is normally done several times as the work progresses.
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Okay, so you're using a syringe and a sealing plate with one hole and you've never done it before and you think my talk about attention to detail is all bullshit so you squirt them just once and come up four measurements ranging from 55cc to 60cc. Now what do you do?
What you do is go back and do it again :-)
When you're satisfied that your measurements are as accurate as they're going to get, the next step is to decide on a strategy for increasing the volume of the small chambers to bring them closer to the volume of the largest chamber. By this time you should have some idea as to the physical size of one cubic centimeter and your measurements have told you how much each of the three smallest chambers must be opened up to match the volume of the largest. The tricky bit is where to remove the metal from.
Basic starting point is unshroud the valves using rotary files. Then do a quick syringe-check. You want to STOP with the filing when you get to within 1 cc of your goal.
If unshrouding the valves does not give you enough volume then you begin ‘laying-down' the edge of the combustion chamber. Here again, start with your rotary files. Try to visualize the amount of metal you want to remove. Remove nearly that amount then check the volume. When you get to within 1cc, stop using rotary files and shift to using abrasive rolls.
The principle behind smoothing things with abrasives is to simply remove the tool marks from previous steps. That means using a tool (or abrasive) that will leave smaller marks. So you start with coarse sandpaper and when you've removed all of the marks of the rotary files you'll be left with the marks made by the sandpaper. So you shift to a finer grade of sandpaper and do it all over again. I generally use 80, 120 & 220. After 220 I shift to felt hobs and polishing compound.
A key point here is that I never try to hit the target volume dead-on, I only try to get close to it and always on the high side. A second point is the need to periodically check the chamber volume as the work progresses... and to check it with an increasing degree of accuracy.
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Running the 4-Minute Mile
The only practical way to achieve 0.1cc accuracy across eight chambers (ie, two pair of heads) is to get close to the target figure, say within 0.5cc, and to then drop one of the valves. That is, to lower one of the valves by grinding it's seat. The area of the valve is fixed; you can measure it. You then determine how many thousandths of an inch it needs to be lowered to arrive at your target volume.
Since most of you don't have the stones, tools and fixtures for re-grinding valve seats, I don't expect you to be using this procedure but I think the how-to is worth mentioning.
-Bob Hoover
(From the unpublished manuscript 'How to Build A Reliable Aircraft Engine')
Most folks arrive at this point not because they are building an engine but because they are repairing one. Doing a valve job on a VW engine alters the chamber volume. Or they've dropped a valve and have had to replace one of the heads. Replacement or repair of a head alters the chamber volume and chamber volume is a factor in the compression-ratio equation. Before they can re-assemble the engine they have to measure the chamber volume and re-compute the CR to ensure the repair work has not upset the compression ratio.
Measuring chamber volume is easy to do and the procedure is all the real service manuals for the VW engine. But just to keep all the information in one place, I'll give you a quick sketch of the procedure.
Using the catheter-type syringe and a sealing disk similar to those shown in the photos, you should be able to measure the volume of a Volkswagen combustion chamber to within 2cc. That is, if you measure it a multiple number of times your results should not vary from the average result by more than plus or minus 1cc. (As a general rule, always measure each chamber at least three times.)
Herez how: Install a spark plug in the head and torque to spec. If the valves are not perfectly fluid-tight, pop them out and put a light smear of Vaseline on their sealing surfaces. Level the head in both directions then smear a light coat of Vaseline around the periphery of the sealing disk and drop it into the chamber. If the sealing disk is 3/8" or thicker it will be heavy enough to stay in place by itself (the specific gravity of cast acrylic sheet is only 1.19). For thinner sealing disks you'll need to hold them in place with your thumb while filling the chamber or add a few ounces of weight to their edges.
The disk(s) doesn't have to be perfectly round. You can saw the thing out with a jig saw, smooth the edge with a file and you've kept it to within +/- sixty thou or so, it will work perfectly well. Nor do you need five holes in the disk as shown in the factory workshop manual; you can do a quick check with just one.
Using a suitable fluid, fill the chamber and the hole through the sealing disk. Subtract the volume of the hole from the total. Do both chambers. For OHV engines I think you'll find ‘wetter' water does a better job than the traditional kerosene. This is because you need to measure the chamber several times and average the result and that means you have to remove all of the fluid from the chamber. With a flat-head engine it's pretty easy to simply wipe the chamber dry but with an OHV the kerosene tends to cling to all the nooks & crannies. So use water with a drop of wetting agent as your fluid. When it comes time to dry the chamber, simply dump it out, slosh it with a bit of alcohol then blow it dry. Alcohol is hygroscopic - - it will mix with any remaining water and once mixed with alcohol it takes only a modest blast of air to dry even the deep recess around the spark plug's insulator.
A quick check using the syringe shown in the photos will allow you to determine the volume of your combustion chambers to within 2cc (i.e., plus or minus 1cc). To compute your compression ratio use the largest volume you measured as the factor in the CR equation.
That's it. If you're a shade-tree mechanic you're all done. Good luck in the contest.
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CC'ing TOOLS & EQUIPMENT
Why am I doing this?
Truth is, some of you aren't. Although this article was written to explain how to improve your engine's efficiency by adjusting it's volumetric balance, many builders are interested only in determining their compression ratio. If that's all you're here for, drop down to MEASURING CHAMBER VOLUME.
By applying modern-day standards of balancing, of both mass and volume, you can pick up as many as ten free horsepower from a bone stock VW engine. And I mean ‘free' in the sense that it will use the same amount of gas. That seeming impossibility reflects the fact the engine must first overcome any internal imbalance before it can deliver any usable power. That means an imbalance generating a loss of five horsepower costs an additional five horsepower to balance the books, resulting in a net loss of ten horsepower. This is a key factor in the importance of balancing and one most people overlook. That is, for each imbalance you eliminate you will gain approximately twice that amount of improvement.
Of even more importance to builders of flying Volkswagens is the fact that a properly balanced engine is more durable and the reduction in wear is even more pronounced than the improvement in fuel efficiency. That's because most of the internal imbalances appeared in the form of friction and heat, which could only be overcome by generating more friction and heat. The bottom line is that a 3hp imbalance could cost you up to four times that in additional wear. Eliminate the imbalances and the engine's wear-factor takes a dramatic drop.
The benefits of mass-balancing became self-evident as the normal operating speed of internal combustion engines was increased but the role of volumetric balance was not fully appreciated until computers came along. Once they had a super-computer to play with automotive engineers discovered that a lot of what we thought we knew about the process of combustion was not entirely correct, that there were transient phenomenons due to volumetric imbalances that we'd attributed to other causes. This is one of those non-intuitive kinds of things, not easily explained without a bit of background but the basic reason has to do with the process of combustion and the fact those seemingly insignificant differences in volume appear on both sides of the compression ratio equation. Turns out, relatively small variations in volumetric balance can produce some relatively large losses of power. Since that time they've taken exquisite pains to reduce those imbalances. How this is accomplished in a mass-production environment is quite interesting (at least, to me :-) but the bottom line is that we can take advantage of what they've learned by simply paying more attention to the volumetric balance of the engines we build.
Thanks to computerized equipment a modern balance shop can do mass-balancing to a fraction of a gram at a very reasonable price. But matching the volume of a pair of heads (i.e., four chambers) to a tenth of a cubic centimeter remains more art than science. In the racing world it isn't uncommon to pay a thousand dollars for a pair of full-trick heads, their volumes accurate to 0.1cc across all four chambers. The cost isn't in the tooling; it doesn't take a lot of tools for head-work. The money goes to buy the head-man's time. That fact is in our favor because time is about the only resource of which lo-buck homebuilders have a surplus. By acquiring a few tools and devoting a bit of time to your heads, you can bootstrap yourself into a better engine than you can afford to buy.
At this point a majority of non-mechanic home-builders are sitting there with a large black question-mark floating in the air over their heads. If adjusting chamber volume is more art than science, how can they hope to tackle the job themselves?
The answer is so simple it will make you smile. Anything you do to reduce the magnitude of the existing imbalance will improve the efficiency and durability of your engine. The fact the pro's regularly balance to a tenth of a cubic centimeter is like running the four minute mile. But we're just a bunch of week-end joggers so let's not even think about that degree of precision. At least, for now. Instead, let's look at the basic problem.
You have four chambers. You measure them (the procedure is outlined below) and come up with four values, probably accurate to within half a cc or so. Using real numbers now... two heads measured 56.5, 58, 58, and 60cc. Rather than trying to turn you into a head-flow guru capable of achieving 0.1cc accuracy on a repeatable basis the question becomes: "Can we reduce that 3.5cc spread to something smaller?"
The answer is a decided ‘Yes!' And it really isn't all that difficult because you only have to work on three of the chambers. That is, you can't make a chamber any smaller so the largest chamber becomes your target-size. So relax. You can do this. And you'll end up with a better engine because of it.
Then comes the question, how much metal are we actually talking about? Just what the hell is a ‘cubic centimeter'? The answer to both: Not much.
If you've been following the Practice Wing project you should have some high-density urethane foam on hand. Print out the ONE_CC drawing, glue it to a piece of foam then use a razor blade to make a cube the size of the printed square. That is, a cube 0.3937" on a side. No foam? Then make the cube out of wood. Or plastic. Or tool steel (if you can). The goal is to give you a tangible reference to the volume of a cubic centimeter.
Hold a cubic centimeter in your hand and three things are immediately apparent: Firstly, it isn't very large. Just a tad over 3/8" on a side. Secondly, you can appreciate the heroics it takes to achieve repeatable tenth-cc accuracy. And finally, you can see why a lot of people consider such imbalances to be insignificant. To the uninitiated, something that small has to be unimportant; something they can ignore.
Got some sixteenth-inch ply? Okay, howz about some poster board a sixteenth of an inch thick. Or a piece of .063 aluminum. Three-eighths wide, six inches long, you're looking at about one cc. Now go back and look at what we're trying to do. To balance the 56.5cc chamber we need to remove three and a half cc's of material. Just unshrouding the valves is usually good for that volume.
So how do we do all that? I'll get into that in a minute but it all begins by measuring the volume of your combustion chambers.
MEASURING CHAMBER VOLUME
If you've never cc'd your chambers, relax; it isn't difficult nor do you need a lot of equipment.
My first exposure to cc'ing heads was in the mid-1950's. I bolted a piece of plex to the freshly milled head of a Ford V-8, leveled the thing up with wedges and used a turkey baster to fill each chamber with kerosene. After doing both heads I plugged the smallest volume into the compression-ratio equation and came up with a CR of about 11:1, assuming I used a sixty-thou gasket. Life was good, assuming I could swipe enough avgas for a couple of runs through the lights :-)
Determining the volume of the combustion chambers is a chore common to all engines and the basic procedure is the same for all. The procedure requires:
1. Some means of holding the head level.
2. Some means of sealing the space to be measured.
3. A method of accurately measuring the amount of fluid needed to fill the chamber.
Working with nothing more than a 1/4" drill motor and a couple of carbon steel rotary files, cleaning up the marks with a strip of sandpaper spiraled around the split end of a quarter-inch dowel, you can expect to produce a cc-job accurate to about 0.5cc across all four heads. Indeed, the usual problem when a novice does his first set of heads is going too far, opening up the smaller jugs to a volume larger than the target-jug. Which means they have to take a bit out of the big chamber, which usually leads the them chasing the volume back & forth until they get sick of it.
SAME PRINCIPLE, DIFFERENT PROCEDURE
The secret to hitting your chamber volume dead-on is exactly the same as for hand-cutting a piece of aluminum to a precise dimension in that you don't shoot for the exact dimension. You always give yourself a clean-up allowance. Cutting aluminum by hand you snip or saw the thing about forty-thou over-size then dress the edge with a file. You do much the same when cc'ing your chambers in that you use the rotary files to get close to the finished size then begin smoothing things up with abrasives, checking the volume periodically.
I mention this now because the next subject has to do with the precision of your cc'ing job and the fact balancing your chambers to a fraction of a cc might take you a couple of weeks (!) if you've never done it before, whereas balancing them to 1cc (across all four) should take only a few hours. If you don't want to spend the time then there's no need to spend the money for an accurate liquid dispenser, such as the one shown in the photos, which is graduated to 0.1cc. Turkey baster or the big syringe, you can work down to 1.0cc with fair accuracy. And there's a big difference in price between a surplus syringe and a certified-accurate, lab-grade burette.
All engines get their chambers cc'd, not just Volkswagens. The cc'ing of combustion chambers is illustrated and talked about in the various manuals on engine building. The three key elements mentioned above reflect the basic core knowledge for any cc'ing job. Then comes a host of details; the How-To stuff, most of which is engine-specific but some of the common factors address the accuracy of the finished job, the degree of precision, how long it takes and various convenience factors. As with most other aspects of proper engine fabrication, cc'ing your heads calls for a keen attention to detail. Most of the time.
The point is that you really don't need much to do a quick check.
Lots of after-market retailers will sell you a ‘cc kit.' It usually consists of a plastic syringe like the one shown in the pictures plus a plastic disk with a hole in the middle.
What's it cost? Hard to believe but some outfits want as much as thirty bucks for a cc kit.
See the syringe in the photos? It comes from a veterinary supply house. New, quantity one, you're looking at three or four dollars. But syringes have a certain shelf life and once they're past it, the supply house often flogs the stuff off as surplus, out-dated or whatever. Buy them a dozen at a time, the syringe shown cost about half a dollar.
The plastic sealing disk is any sort of flat plastic, eighth of an inch thick or more. Doesn't have to be clear. The sealing disks I use for 77mm jugs are made from a hunk of red Plexiglas I dug out of the scrap box at a plastics retailer. In a pinch I've even cast my own using polyester resin on a sheet of waxed glass. It only has to be flat on one side and just clear enough so you can chase the bubbles. Chasing the bubbles is one of those little details no one mentions :-) The bitter truth is that some bubbles don't like to be chased and you can spend the best years of your life trying to coax the thing over to the hole.
The sealing disk in the typical kit has just a single hole in the middle. Cheap, easy to make and so forth. But harder to use than one with more holes. More holes, you don't have to chase the bubbles so far but more importantly, with more holes there's a better chance you'll be working with a truly level head. And that's worth mentioning because a lot of after-market heads, especially rebuilts, the sealing surface is not parallel to the valve gallery gasket rail, which is what you use to sit the head on.
So you sit the head on the gasket rail (after taken the studs & valve stems into account) and use your machinist's level or framing level or whatever on the chamber-side of the head to use wedges and so forth to get everything perfectly level, end-to-end and fore & aft.
That's no guarantee the sealing surface of the chamber is level. But if your sealing disk has a pattern of holes as shown in the drawing (and in the VW factory workshop manual), as the chamber fills the level of the fluid in the various holes tells you if the chamber is level.
You subtract the volume of the holes, by the way. Figure out their volume ahead of time, scribe it right onto the sealing disk. You can use the same equation as for V1, just substitute hole diameter for bore and the thickness of the plastic for stroke. Then multiply that by however many holes you have and be sure to always fill the hole to precisely the same level.
Buy a cc kit, it usually comes with only one disk for each size of cylinder. Indeed, some outfits only sell you one disk, period, screwing you to the cash register if you want two disks or a set of a different size. Which is another example of how VW after-market retailers screw the kiddies, because to do the job properly you really should do both chambers at the same time... and that means two disks of each size.
Making disks is pretty simple, assuming you understand plastic. Drills for use with plastic looks like a needle :-) We usta call them ‘canopy bits' or ‘Plex bits.' Included angle of something like 30 degrees. If you don't have a set of bits for plastic and don't know how to sharpen a drill bit, you'll have to use a hand-drill. Even then, you're liable to crack it when the bit breaks through the other side. That's when it grabs the flutes. Plastic bit, you needle a pilot hole then flip it over, finish it from the other side. Slow speed, always. Kerosene makes a good lubricant for thicker stock.
Although the sealing disk drawing I've posted shows dimensions to thousandths of an inch that's just the math. You can cut one out with a jig saw and it'll work just fine. That's because the sealing lip inside the combustion chamber is at least a tenth of an inch wide. So long as the disk is close to a true circle and makes good contact with that lip, it will work okay. (Too big? Then sand the edge.)
Once you understand the purpose of the sealing disk you'll see them all around you, just waiting to be cut out. See that translucent plastic over the waterproof light in the bathroom? That'll work. You can still see the bubble. Got some curvy scraps from an a blown canopy? That'll work. Just put it on a heavy plate of polished aluminum, pop it in the oven set at about 300 and let it flow out flat.
Want to keep your sealing disks nice & pretty? Got some old 5-1/4" floppy disks? Cut one open, throw away the disk, store your sealing disks in the sleeve.
To make your chamber water-tight you smear a little Vaseline on the valve seats and a light wipe around the outer-most edge of the plastic disk. Then you fill the syringe to a precise level and use it to fill the chamber. Subtract what's left in the syringe from whatever you started with, then subtract the volume of the hole(s) in the plate and that's the volume of your combustion chamber, accurate to however accurate you are and as precise as the divisions on your syringe.
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On the other hand... there's lots of lab equipment scaled to 0.1cc increments but most of it lacks the volume to fill a chamber. VW combustion chamber volume may be as small as 40cc or as large as 80cc, depending the size of your valves and the displacement of the engine. A burette large enough to hold one chamber's-worth of juice and accurately marked to 0.1cc precision, you're talking about an expensive piece of goods. The high price of the Good Stuff comes as a shock to guys building just a single engine, often causing them to settle for half a loaf, using a turkey baster of whatever.
So use a marble or two. And a smaller but more precisely marked burette.
Got a marble? There's a free one in every can of rattle-can paint. (That's the rattle :-) Or you can buy a bag of them at the dime store. Or use steel ball bearings (but remember they rust). I like ball bearings because of their uniformity. Marbles vary by quite a bit.
So what's the volume of your marble? Half fill your burette, bring the meniscus to the line, make a note, then drop your marble into the burette. Make another note then do the math. (You may shout ‘Eureka!' if you wish :-)
By adding objects of known volume to your combustion chamber you can use a smaller dispenser marked to 0.1cc graduations. Just be sure to keep the numbers straight. (KEEP GOOD NOTES!).
When trying to achieve the smallest possible difference across all four jugs, statistics are your friend. Make all of your measurements a number of times and then average them. This helps to reduce the human error, a factor that can be deadly for the first-timer. Without experience you can't appreciate the significance of seemingly minor differences. Do everything several times, throw out the high and the low then averaging the remainder tends to reduce any errors of observation.
Work with good light. Don't be shy about using a reading glass to inspect the meniscus in the sealing plate holes and your burette. When taking a series of measures on the same chamber(s) always be sure to start with it perfectly dry. That means you'll need an air blast to blow out the spark plug (which should be torqued to spec on a new washer). Before filling the chambers I swab them with lacquer thinner or MEK to remove any oily residue that might prevent the fluid from fully wetting the surface. When applying the Vaseline, don't get sloppy -- a little dab will do ya.
Once the sealing disk is in place, it don't wanna leave :-) Use a dental tool or bend a tiny hook on the end of a piece of wire to lift it out using one of the holes. Pour the water out of the chambers, wipe them down with a clean towel, give it a slosh of alcohol and blow the plug dry.
Traditionally, the liquid used for cc'ing heads was kerosene. (I donno... it just was.) Working with VW's I've heard guys advocate the use of everything from anti-freeze to ATF and big dummy that I am, I tried them all.
Hell of a mess.
Light oil of any kind is a contaminant in the shop and can be a fire hazard. And glycol, Prestone or what-have-you is poison. In the mid-70's I started using ‘wetter' water for cc'ing and found it gave more consistent results and a lot less mess. How do you make water wetter? Originally, I added a couple drops of ‘Photo Flow,' darkroom stuff that prevents water spots on your negatives. But for cc'ing heads, a couple of drops of liquid detergent per gallon of water has about the same effect. A drop or two of food coloring will make it easier to see in the burette. Try to work in a room-temperature environment. The temperature of your measuring fluid and the heads should always be the same. (Keep the liquid in the shop with the heads. You'll need to do rough spot-checks as the works progresses. I'll have more to say about that in a minute.)
I normally use a burette similar to the one in the photos. You'll notice that it's attached to a standard that bolts to the base, which is a plywood box filled with concrete. To attach the burette to the standard, fancy name for a scrap of plywood, I twisted some welding rod around a bolt a little smaller in diameter than the burette so the resulting hair-pin-looking piece would grip the burette. To fasten it to the standard I drilled a pair of holes, poked the legs of the hair pins through the holes, bent them over and covered the bent ends with a slather of Bondo. (You can see all this in the pictures.)
The base is heavy to prevent it from tipping over and breaking the burette. It is leveled with shims or screws then the burette is over-filled using a pitcher and funnel. To dispense the liquid I use a length of surgical rubber tubing (drug stores carry it) fitted with the glass tip from an eye-dropper (American Science & Surplus (www.sciplus.com) sells all this stuff, including a pretty good Chinese burette). Flow is controlled by a clamp on the rubber hose. Once the burette is over-filled I bring the fluid down to the line by releasing the clamp and returning the fluid to the pitcher or bucket.
To fill the chamber I hold the eye-dropper/nozzle over one of the holes then play with the flow-control clamp until I bring the fluid level to the base of the sealing disk then chase out any bubbles by tapping on the disk with my fingernail or the handle of a dental tool. Once all the bubbles are accounted for I fill the central hole a drop at a time until all of the holes are at precisely the same. Then I read the burette, hunkering up or down to eliminate any parallax.
Measuring a chamber takes me less time than it took you to read the paragraph above. Using two disks allows me to do both chambers with one set-up.
Then I clean everything up and do it all over again.
To determine the finished volume, as when I'm down to the point of making fractional cc adjustments, I measure each chamber as many times as I think I have to. Sometimes I'll get three identical measurements in a row on both chambers. And sometimes I'll do it half a dozen times and get a different measurement each time, usually when I'm tired or whatever. (People wonder why I don't answer the phone or throw rocks at them when they arrive unannounced. Now you know :-)
That's how I do it when I'm balancing the volume of the heads, trying to achieve 1cc or better across all four. When I'm in the shop working on the heads, I generally use a syringe. It's only accurate to 2cc but it's a lot faster and when you are first opening up the heads, 2cc accuracy is more than enough.
- - - - - - - - - - - - - - - - - - - - - - - - - - -
When building a big-bore engine, cc'ing your heads begins long before you get around to calculating your compression ratio. In fact, it starts when you first acquire the heads. If they're new, with valves already installed, just install a spark plug, grease them up and do a rough measure of volume. With bare heads or heads that have just been overhauled (ie, welded-up with new seats, etc.), rough in the valves then install a plug. Level them up, select the appropriate sealing disk (if you haven't opened them up they'll be some other size) and use a baster or syringe to get some idea of their as-cast volume. It's handy to write that volume directly onto the head using a crayon or wax pencil. The heads should already have been documented and assigned a work number and a documentation package (sounds complicated; actually, each head is just a page in a notebook.) A lot of folks think that's overkill for just one engine but during the course of the work you must have some means of identifying the chambers and even one engine has four of those suckers in two identical heads. If you're building a really good engine, you'll make up a set of spare heads, identical to the first set. Having a spare set of heads on-hand allows you to swap heads when necessary and do the valve work at your convenience.
When you open up your heads for bigger jugs you are trying compress the swept volume of the larger jug into the original compression space. There's no doubt the compression ratio will go up. The big question, is how much?
If the only thing that changed was the swept volume and the deck clearance was kept the same, going from 1600cc to 1834 is going to raise your CR by about one point. But that's the myth that leads to blown engines because nothing remains unchanged. When you open up the heads even the slightest skim cut will reduce the chamber volume. Another part of the myth is that most guys don't know what their original compression ratio was to begin with. Later model engines ran 7.5 or 7.7 (the Export crate engine from the Puebla plant is 7.7) depending on the heads but if it's been overhauled a few times you could be using heads that have been flycut. Bottom line: It isn't uncommon to see 1834's with a CR of 9:1 or more.
Opening up your heads for bigger jugs offers the opportunity to unshround the valves. Unshrouding the valves results in a profound increase in flow-rate for the same amount of lift. The procedure is covered in Bill Fisher's "How to Hot-rod Volkswagen Engines" but I'll try to insert a couple of photos to show you what I'm talking about.
To unshroud your valves you have to remove metal from the chamber. Any time you remove metal from the chamber you need to cc the chambers to make sure they all end up the same size. This is normally done several times as the work progresses.
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Okay, so you're using a syringe and a sealing plate with one hole and you've never done it before and you think my talk about attention to detail is all bullshit so you squirt them just once and come up four measurements ranging from 55cc to 60cc. Now what do you do?
What you do is go back and do it again :-)
When you're satisfied that your measurements are as accurate as they're going to get, the next step is to decide on a strategy for increasing the volume of the small chambers to bring them closer to the volume of the largest chamber. By this time you should have some idea as to the physical size of one cubic centimeter and your measurements have told you how much each of the three smallest chambers must be opened up to match the volume of the largest. The tricky bit is where to remove the metal from.
Basic starting point is unshroud the valves using rotary files. Then do a quick syringe-check. You want to STOP with the filing when you get to within 1 cc of your goal.
If unshrouding the valves does not give you enough volume then you begin ‘laying-down' the edge of the combustion chamber. Here again, start with your rotary files. Try to visualize the amount of metal you want to remove. Remove nearly that amount then check the volume. When you get to within 1cc, stop using rotary files and shift to using abrasive rolls.
The principle behind smoothing things with abrasives is to simply remove the tool marks from previous steps. That means using a tool (or abrasive) that will leave smaller marks. So you start with coarse sandpaper and when you've removed all of the marks of the rotary files you'll be left with the marks made by the sandpaper. So you shift to a finer grade of sandpaper and do it all over again. I generally use 80, 120 & 220. After 220 I shift to felt hobs and polishing compound.
A key point here is that I never try to hit the target volume dead-on, I only try to get close to it and always on the high side. A second point is the need to periodically check the chamber volume as the work progresses... and to check it with an increasing degree of accuracy.
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Running the 4-Minute Mile
The only practical way to achieve 0.1cc accuracy across eight chambers (ie, two pair of heads) is to get close to the target figure, say within 0.5cc, and to then drop one of the valves. That is, to lower one of the valves by grinding it's seat. The area of the valve is fixed; you can measure it. You then determine how many thousandths of an inch it needs to be lowered to arrive at your target volume.
Since most of you don't have the stones, tools and fixtures for re-grinding valve seats, I don't expect you to be using this procedure but I think the how-to is worth mentioning.
-Bob Hoover
Sunday, December 3, 2006
Leaky Oil Seal
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Kid comes in the shop wringing his hands, all upset. He's just paid a lot of money for someone to rebuild the engine in his bug and it's leaking oil from the front seal. The guy who did the engine sez it's not his fault, it must be coming from the rear tranny seal, wants to sell the kid a rebuilt tranny. And besides, all VW's drip oil; no big deal. But the kid is sure it's leaking more than it did before it was overhauled and he's never had his tranny leak and the leaky stuff doesn't have that thick sulphury smell like tranny lube and his mom is all upset about the gunk on the driveway.
That last is the real reason he's here :-)
So you drop the engine, pull the old seal, check the end-play ('way off), clean the oil drain drilling, put in a new shim stack and a new oil seal. To further the kid's education you show him how the seal has been ruined; whoever installed it used a hammer.
Hammer = Drips
At least half the drippy oil seals I see are caused by improper installation of the seal. A few are due to improper engine assembly. The remainder are from excessive end-float; the thrust face of the #1 bearing is simply worn out, allowing the crankshaft to move back & forth. When it does, it acts as a nice oil pump, defeating the purpose of the lip-type seal.
The #1 main bearing oil seal is neoprene or silicone rubber bonded to a metal ring with a circular coiled spring inside to maintain a leak-free sliding fit around the center boss of the flywheel. The oil seal fits into a recess cast into the crankcase; it's a tight fit. Properly installed, the oil seal ends up slightly below the level of the casting.
The factory service manual shows the seal being pressed into place using a screw-type fixture, but an experienced mechanic can install one using repeated light taps from a plastic mallet. An unskilled mechanic will try to do it with a hammer and while it might look okay, about half the time the hammer blows cut the silicone rubber where it's molded over the invisible metal ring inside of the oil seal. Oil quickly discovers the cuts and you've got a leaker on your hands.
Too Much Sealant = Drips
A leaky seal due to improper engine assembly is a bit more subtle.
Like all of the main bearings, #1 is generously supplied with pressured oil form the main oil gallery. In normal operation the oil lubricates the journal and escapes from both sides of the bearing. On the flywheel-side of the bearing the oil collects between the bearing and oil seal, flowing back to the sump via a drilling in the left half of the crankcase. Want to guess what happens if that drilling gets blocked? (Be careful, it's a trick question.)
When the #1 main bearing oil return is blocked, oil pressure will build up behind the seal and the thing will eventually leak. That's the obvious answer. But a blocked oil return port also results in accelerated wear since the thrust face of the bearing and the associated shim stack is not being provided with a circulating supply of oil.
Most often, the oil return passage is blocked by an over zealous application of sealant when the crankcase halves are joined. Here's how it happens: The left half of the crankcase is in the fixture, parting line up, the guy swabs on about four times more sealant than needed and when he drops the right half of the crankcase into place it squeezes the sealant out, which flows downhill into the oil return passage from the #1 main bearing. And that's just on the inside of the crankcase. Outside, the sealant is oozing all over, including down into the recess for the oil seal. Being in the 'corner' of the recess, the oil return passage gets more than its share when it shouldn't have gotten any at all. (Hint: After closing the crankcase for the last time, check the oil return drilling with a Q-tip to ensure it is clear.)
I've also seen engines with sealant deliberately painted into the oil seal seat, apparently hoping to stave off leaks. In those cases the oil seal itself was always damaged by hammer blows.
Think about that for a minute. The guy builds an engine, hammers in the oil seal and sure enough, the puppy leaks like a sieve. So the next time he globs on a lot of sealant, hammers in another oil seal and this time it leaks even worse, convincing him it's impossible to keep a VW engine from leaking. (And besides, everyone sez VW's leak. Conventional Wisdom wins again.)
Indeed, when you combine an improperly installed oil seal with an improperly assembled engine (ie, the blocked drain hole) the engine doesn't just leak, it gushes. In effect, the builder has just created a direct path from the oil pump to the ground under the engine.
So why do people install oil seals with a hammer? First, because they see a real mechanic do it successfully and never understand that it takes considerable skill to do it right. Secondly, they do it because most of the manuals say it's okay to hammer it in... and about half the manuals show the seal installed incorrectly, flush to the outside of the crankcase. Correctly installed, the seal will be slightly below that level.
But the most common reason for all those drippy engines is the fact everyone assumes that hammering requires no skill.
Push or Pull = No Drips
Oil seals are designed to be pressed or pulled into their seats. It's possible for an experienced mechanic to install them with a plastic mallet, or even a hammer in the case of some axle seals, but it's also possible for a skilled surgeon to do an appendectomy with a pocket-knife. The emphasis here is on the skill, not the tools.
Oil seals aren't expensive and they don't look very sophisticated but there's more to them than meets the eye. If you toss an old one on the barby and wait for a while you'll be able to examine what's under the rubber. You'll see that most of them start out as a segmented ring of thin sharp steel. Cover that with rubber, tap on it with a hammer and it cuts the rubber as neatly as a knife.
So press them in. Or pull them. You can make a dandy puller-presser for your front brake drums (the seals you'll replace most often) using nothing more than a length of all-thread, some washers and three nuts. And you can buy a screw-type pressor for the #1 main bearing seal, although it's easy enough to make one, assuming you have a lathe. (See the drawing at the top of this article.)
Or you may drive them in with a 'seal-seater,' if the seal is small. By distributing the force of the hammer blows uniformly, a seal driver lets you pop the things into place with one or two well placed blows of a hammer. If you've a lathe, making oil seal drivers is a spare-time sorta thing; all are simple turnings, and aluminum or even hardwood works as well as steel.
Big seals are different. Because of their tendency to cock in the bore, large-diameter seals are best installed with a press or fixture. Rear axle seals are especially troublesome due to their deeply recessed position in the seal cover. Because of their proximity to the brakes and the fact that any leak could leave you without brakes, the wiser course is to always press-in rear axle seals.
Sermonette
With the exception of the Muir manual, books on maintaining your Volkswagen assume a certain level of competence. Learning to tap a seal into place with a plastic mallet isn't difficult; it's one of the many minor skills acquired during the apprenticeship all mechanics must endure. It is also one of the minor skills many self-taught mechanics never bother to master. (Hint: Start with an old seal. And an old engine case. When you can tap the thing in a dozen times in a row without damaging either the case or the seal you're probably ready to try it with a new seal on a good case. Along the way, you will have learned how to remove the thing as well.) But installing a seal with a hammer falls into the category of 'Field Repairs;' things a skilled mechanic must do when the proper tools are not available. Pressing the seal into place is not only safer, it's usually faster. And a pressed-into-place seal is cheap insurance against oil leaks.
Copyright © 1995 Robert S. Hoover
One Engine's-Worth of Parts
In response to one of Rocky's messages I mentioned a number of things that can effect compression ratio. Sunday I go to check the mail and there's this buncha guys peering in my window all saying pretty much the same thing:
I don't see how ... (you fill in the blank) can have any effect on CR.
A minor variation on the theme was:
(Your favorite expert's name goes here)... sez to do it like ( whatever) and never mentions (...various unmentionables...).
Please accept the following as a general answer for all.
CRANKCASE
The four holes in the crankcase that accept the cylinder barrels are called spigot bores. The area around each bore is called the deck and serves to support the cylinder. The decks of all four spigot bores must be the same distance from the center-line of the crankshaft. This is something you check before you start building any VW engine even when using a new crankcase because sometimes the axis of the crankshaft is machined slightly eccentric, meaning the main bearing bores are a little bit deeper in one half of the crankcase than the other. Or more rarely, machined at a slight angle, with the clutch-end being more to the left, the pulley-end to the right (or visa-versa). Not often but it happens. So you check it.
With any used crankcase the spigot bore decks will have been re-faced -- re-machined to get rid of the shuffle marks. Good shops with the right equipment always machine the case decks so all four will match but if you buy a used crankcase from a shade-tree mechanic or a shop that caters to the kiddie trade you're liable to find almost anything. I've seen cases with as much as sixty thou variation in the spigot deck height from one side to the other... and almost that much on the same side of some cases, which tells you the case came from a drill-press operation (ie, a shop that doesn't have a milling machine).
Your jugs sit on the deck around the spigot bores. If there is any difference in their height it will be reflected in the height of the cylinders. And since the con-rod extension is relative to the center-line of the crankcase, any variation in the height of the cylinders will show up as a difference in the deck-height of the piston at TDC.
And that will effect your Compression Ratio.
If that's not clear, make a drawing and work it out but the message here is that you have to know what your case-deck-height is. You can't guess. You need to blueprint the case and record your findings, whatever they are, because you're about to build on that foundation and by the time you get out to the heads you will have stacked up half a dozen components and even the smallest variations will have become significant because of the stack-up.
Major point here is that there is always some amount of deviation from spec in the parts going into your engine. With an army of inspectors to insure the quality of every step in the manufacturing process, for original Volkswagen parts the variations would tend to cancel each other out rather than stack up. That's not true with after-market parts. The only way to know what you have is to measure what you got. Some guys call this 'blueprinting' and make a big deal out of it but it's mostly common sense.
CRANKSHAFT
Set the crank up in vee blocks or with fitted bearings in a known-true case half and check the length of the throws, even if it's a good crank you've just sent out for a polish. Sometimes the grinder will have a bad day and you'll end up with a crank having a slightly different stroke on one (or more!) of the journals. So you check it to within the accuracy of your tooling and record the results. Usually, cranks are pretty good. Some of those cranks coming in from China are as good as any I've seen. But some are trash. Ditto for a lot of welded strokers aimed at the Kiddie Trade, with examples of every problem you can name being woefully common. You have to check and record what you find even when any variation falls within acceptable limits because that variation, whatever it is, will add to or subtract from the finished dimensions of the engine.
CONNECTING RODS
To ‘rebuild' a rod you re-bush the little end, hone the bush to spec then pull apart the big end, use a surface grinder to remove a little metal from the parting line, torque it back together and machine the big-end back to a true circle relative to the little end. That is, you try to keep the distance between the center of the big end to the center of the little end the same as for a new rod fresh from the factory.
Sunnen (brand name) honer that has been properly maintained, skilled machinist... you can produce a pretty good rod. Shops that cater to the kiddie trade... wetback labor... worn-out or poorly maintained machine tools... Forget about it.
So what's the spec for a stock length rod? I donno... 137mm? Something like that.
Doesn't really matter. (!!) What matters is that all four of your rods must be of identical length. That's what matters. Long or short, you can deal with that but only if they are all the same.
But they won't be. There will be some variation in their center-to-center length, center of mass and over-all mass. You'll take care of the weigh differences during balancing but right now you need to know the variation in their center-to-center length, which is pretty easy to measure even with simple tools if you use one journal of a crankcase as your center on the big end and a well fitted wrist pin on the other.
Con-rods are numbered. Use their number in your records when you record the difference in their lengths. SOP is to identify the shortest rod then simply record the differences of the other three as 'pluses.' Good rods, you'll be working in tenths... +8, +4 (ie, +0.0008, +0.0004, etc.)
What's a well fitted wrist pin? Oiled and at room temperature, you should be able to slide the pin into the little-end with your hands. Once in, it should fit well enough so that the pin takes at least two or three seconds to slide out when the rod is held horizontally (and the pin is installed flush). Slower is better. At running temps the rod will expand more than the pin so a good fit is one that is damned tight at room temperature.
A lot of rods aimed at the kiddie trade or used by lo-buck rebuilders aren't even overhauled. They only knurl the bushing then hone it back to size and simply hit the big-end with a hammer before honing, if they bother to hone it at all.
Shop by price, you'll end up buying junk. Good shops are proud of the quality of their work, offer no objection if you want to mike a part now & then. Ditto for good dealers. The other kind don't want anything to do with real mechanics. And get their wish :-)
The point here is that the length of the connecting rods effects the Compression Ratio.
PISTONS & CYLINDERS
Pistons & cylinders are manufactured individually then sorted according to their finished diameter (for jugs) and weight (for pistons). The different sizes and weights are identified by dots of colored paint on the pistons.
In manufacturing a cylinder barrel the raw casting is first machined then the machined barrel is honed to remove the tool marks. In the process of machining a given number of cylinders, the finished bore will become gradually smaller as the tool-bit wears down. When it gets to a certain minimum size they stop the machine and set it back up with a new boring tool. That means the inside diameter of the jugs will fall across a certain range of diameters. This is normal.
The honed jugs are measured and divided into groups according to some standard deviation in their diameter, typically about a thousandth of an inch. But even with that small a standard, with four jugs from the same size-group you can expect to find a variation in their diameter. It won't be much but you need check it.
Volkswagen used cast aluminum pistons from permanent molds. The density of cast aluminum varies slightly according to how much metal is in the smelting pot, its temperature and how long its been there. The castings are then machined to a given diameter, for the grooves where piston rings, for the wrist pin and for the top of the piston. All other surfaces are usually left as-cast. As with all machining operations, the finished dimensions will fall across a range of sizes.
The combination of differing density in the aluminum alloy and variations in the as-cast dimensions causes VW pistons to vary in weight by as much as an ounce (!) Even by 1930's standards that's a bit much so the pistons get sorted into three weight groups with each group having a maximum variation of ten grams.
The nominal dimension of the piston (i.e., its size group) is stamped on the top and a dot of colored paint is used to indicate which direction its actual dimension deviates from the stamped figure. Another dot of different colored paint is used to indicate the piston's weight group and a plus or minus symbol is stamped into the top of the piston to indicate if the piston's weight is above or below the nominal weight for that group.
The pistons are divided into groups according to their weight and within each weight group, are divided into groups according to their diameter, allowing them to be matched with suitable jugs, fitted with rings and packaged for shipment. Stock jugs used to be available individually; nowadays all you'll see are sets of four.
But your carton of new pistons & cylinders may arrive as a grossly mis-matched set of junk. Here's why: Some after-market retailers -- or the clerks who work for them -- tear open the boxes and shuffle sets around to make up sets having the largest bore diameter and identical weight markings. Some dealers even brag about this in their advertising, referring to such sets as the ‘pick of the litter' that need no further balancing. And sell such sets at inflated prices.
It's all bullshit of course, a minor deception aimed squarely at the Kiddie Trade. Why? Because with a weight group encompassing ten grams, with two divisions and a mark for high or low the best you can hope for is a spread of 2.5g... about 25x worse than a real balancing job. (Using an inexpensive electronic scale for measuring and a Dremel tool for removing metal, the average novice has no trouble matching four pistons to within a gram or two.)
But the most interesting point of all this is what happens after those sets of pistons have been pawed over by the clerks. They get tossed back into the boxes willy-nilly and sold to unsuspecting suckers, including other retailers.
The tricky bit here is that you can't balance a set of pistons if they span two weight groups. Pistons are provided with extra metal in the form of ‘balancing pads,' areas from which you may remove metal without effecting the strength of the piston. But the maximum amount you can remove is only a few grams. That isn't a problem when all of the pistons are from the same weight group. But when your P&C's are a mix of two (or more!) weight groups you're liable to see as much as 20 grams difference across your four 'brand new' jugs. Not only does that violate the factory spec of 10g, the difference is too large to be balanced out - - there simply isn't enough metal that can be safely removed.
You just paid good money for a set of new jugs that are junk.
But this is about compression ratio so let's get back to that.
First thing you gotta do is examine your new set of P&C's to make sure they are of the same size group (ie, the variation of diameter) and within the same weight group. That is, all four of the jugs in the box should have the same color code for dimension and the same color basic color code for weight group. The code for plus & minus doesn't matter because you're going to have them re-balanced to a finer standard of precision (i.e., typically +/- 0.1g across a set of 4).
You should do all that before you buy them. And yes, you can get royally screwed when buying through the mail. No, I won't recommend anyone -- I've been sued both ways on that one, once because a guy was unhappy with someone I recommended and another time by a dealer because I didn't recommend him. So go fish. And good luck. Because getting a set of P&C's that hasn't been tampered with is just the start of the story.
Once you have a set of P&C you'll need to put identifying marks on the jugs and record the marks and the dimensions in your notes. I file notches in the flat area of the upper-most fin. When you have more than one engine in the shop at a time, keeping their parts separate can be a problem. I use a series of adjoining notches to identify the set then one to four additional notches, spaced apart, to identify a particular jug within a set. The notches are cut with die-grinder as soon as I open the box. The pistons have to stay with their particular jug so you need to put a matching mark or number on the underside of that piston. I use a vibrating scriber.
Begin your measurements with the distance between the deck lip and the top of the cylinder barrel. The easy way to do this is to just stand the thing on its head and use a surface gauge to find the tallest barrel then record any difference in the other three. Here again, you can expect some small variation.
Barrel length is an especially critical dimension in an horizontally opposed engine since it is the foundation of the valve train geometry. This dimension is even more important in horizontally opposed engines like the Volkswagen which depend upon head studs (or stays) to maintain the seal between the cylinder and the head since any difference in the length of the barrels will impose an asymmetric load on the sealing surface leading to compression leaks.
After measuring the length of the barrels the pistons are removed and the pin height is measured. Follow the same general procedure; put the piston, head down, on a surface plate, use a gauge to find the tallest then record the difference between it and the other. (As a point of interest, in most cases there's nothing to record - - the dimensions match to within less than a thousandth of an inch and an amount that small is generally not significant. What I'm really looking for here is any radical departure from the norm.) But the fact remains, any dimensional variation in bore diameter, barrel length and piston pin height will have some effect on your Compression Ratio
The rings get removed and bunch of other work gets done but we're only talking CR here so I won't go into the other stuff.
CYLINDER HEADS
As with the jugs, when measuring the heads you must first identify them. Through the course of assembling an engine the heads get a lot of work done to them and you need to keep good records. I stamp numbers on them, over by the right-hand exhaust stack (right-hand looking into the chambers, push-rods down). Doesn't really matter how you identify them just so you do. I use stamped numbers because in prepping a set of heads I usually replace some of the guides, run them through the blasting cabinet to roughen up certain areas then open up the chambers, unshroud the valves and do a few other things, most of which will destroy any kind of temporary markings.
On the chamber-side of the head casting you will find either a fully machined flat area surrounding the chambers (old style heads and some after-market types) or six machined bosses, three to each chamber. The horizontal plane defined by the machined surface, either of the bosses or of the flat area, is the base-line for all of your head dimensions.
You need to know the distance from that horizontal plane to the sealing surface of the combustion chamber. More specifically, you want that distance to be as close to identical as possible for both heads and, within a head, for both chambers.
This dimension can be all over the map if the heads have been opened up by a schlock shop. Good shop, any variation should only be a few tenths (ie, ten-thousandths of an inch) up to a max of half a thou (ie, fifty ten-thousandths). Shlock shop, using a cutter in a drill press, you won't believe the crap they turn out.
This dimension is especially critical in the fabrication of a good VW engine. If this distance varies by more than two thou between the chambers of the same head, or by five thou between a pair of heads, have the heads fly-cut by the minimum amount needed to arrive at a uniform figure for all four chambers.
With measurements for the case deck height, barrel length, rod length and piston head height, and knowing the compression ratio you are planning to use, measuring your chamber volumes tells you how much you will have to open them up to achieve the desired compression ratio. Indeed, once you've nailed down a few dimensions, setting up the correct compression ratio becomes something of a no-brainer.
And somewhere about now you'll realize this message wasn't about compression ratio at all :-)
There are two main reasons for doing the work described above. The first is to be able to identify good parts from bad parts. You can't make this determination by price nor the fact the part is new, rebuilt or whatever. Nowadays there is so much junk out there the wiser course is to assume you're dealing with shoddy goods until its specs prove otherwise.
As you progress through the measurement of the parts you begin to see ways in which you can combine those parts so as to arrive at the most dimensionally-uniform result. For example, a slightly short throw on the crank can be combined with a slightly long rod. The same is true for the jugs and the heads in that some combinations may be used to cancel out dimensional variations.
A nice point to keep in mind here is that the ‘assembly' of a ‘paper' engine is an arm-chair activity. You may take as long you wish, shuffling the numbers about in every possible combination until you arrive the one that makes the best possible use of that particular set of parts.
OF THAT PARTICULAR SET OF PARTS...
Did the light-bulb come on over your head? You see, the typical engine-builder can only afford one set of parts. And as much as I hate to say it, if you simply bolt them together the odds of getting a good engine are vanishingly small. Oh, it'll run. Veedubs are robust little buggers... almost anything will run. But if you simply throw the thing together it will not run as well as it should nor last as long as it could. And you won't know the difference.
But I'm not a machinist... (I heard someone shout).
Neither was W. Edwards Deming. He was a statistician with the Bureau of the Census. (Never heard of him? Your loss.)
The truth is, you don't need to be a machinist to build a better engine. You can do that by simply taking a few measurements and keeping good notes. That's enough to keep you from building a total piece of shit. When you subtract the POS Probability Factor from the engine building equation you automatically end up with a better engine. How much better? On average, about twice as good. Yeah, I know... nobody else believes it either. Except for the guys who have done it. (Didja read my article on dialing in your cam? Ditto.)
Up to you. It's your engine.
-Bob Hoover
I don't see how ... (you fill in the blank) can have any effect on CR.
A minor variation on the theme was:
(Your favorite expert's name goes here)... sez to do it like ( whatever) and never mentions (...various unmentionables...).
Please accept the following as a general answer for all.
CRANKCASE
The four holes in the crankcase that accept the cylinder barrels are called spigot bores. The area around each bore is called the deck and serves to support the cylinder. The decks of all four spigot bores must be the same distance from the center-line of the crankshaft. This is something you check before you start building any VW engine even when using a new crankcase because sometimes the axis of the crankshaft is machined slightly eccentric, meaning the main bearing bores are a little bit deeper in one half of the crankcase than the other. Or more rarely, machined at a slight angle, with the clutch-end being more to the left, the pulley-end to the right (or visa-versa). Not often but it happens. So you check it.
With any used crankcase the spigot bore decks will have been re-faced -- re-machined to get rid of the shuffle marks. Good shops with the right equipment always machine the case decks so all four will match but if you buy a used crankcase from a shade-tree mechanic or a shop that caters to the kiddie trade you're liable to find almost anything. I've seen cases with as much as sixty thou variation in the spigot deck height from one side to the other... and almost that much on the same side of some cases, which tells you the case came from a drill-press operation (ie, a shop that doesn't have a milling machine).
Your jugs sit on the deck around the spigot bores. If there is any difference in their height it will be reflected in the height of the cylinders. And since the con-rod extension is relative to the center-line of the crankcase, any variation in the height of the cylinders will show up as a difference in the deck-height of the piston at TDC.
And that will effect your Compression Ratio.
If that's not clear, make a drawing and work it out but the message here is that you have to know what your case-deck-height is. You can't guess. You need to blueprint the case and record your findings, whatever they are, because you're about to build on that foundation and by the time you get out to the heads you will have stacked up half a dozen components and even the smallest variations will have become significant because of the stack-up.
Major point here is that there is always some amount of deviation from spec in the parts going into your engine. With an army of inspectors to insure the quality of every step in the manufacturing process, for original Volkswagen parts the variations would tend to cancel each other out rather than stack up. That's not true with after-market parts. The only way to know what you have is to measure what you got. Some guys call this 'blueprinting' and make a big deal out of it but it's mostly common sense.
CRANKSHAFT
Set the crank up in vee blocks or with fitted bearings in a known-true case half and check the length of the throws, even if it's a good crank you've just sent out for a polish. Sometimes the grinder will have a bad day and you'll end up with a crank having a slightly different stroke on one (or more!) of the journals. So you check it to within the accuracy of your tooling and record the results. Usually, cranks are pretty good. Some of those cranks coming in from China are as good as any I've seen. But some are trash. Ditto for a lot of welded strokers aimed at the Kiddie Trade, with examples of every problem you can name being woefully common. You have to check and record what you find even when any variation falls within acceptable limits because that variation, whatever it is, will add to or subtract from the finished dimensions of the engine.
CONNECTING RODS
To ‘rebuild' a rod you re-bush the little end, hone the bush to spec then pull apart the big end, use a surface grinder to remove a little metal from the parting line, torque it back together and machine the big-end back to a true circle relative to the little end. That is, you try to keep the distance between the center of the big end to the center of the little end the same as for a new rod fresh from the factory.
Sunnen (brand name) honer that has been properly maintained, skilled machinist... you can produce a pretty good rod. Shops that cater to the kiddie trade... wetback labor... worn-out or poorly maintained machine tools... Forget about it.
So what's the spec for a stock length rod? I donno... 137mm? Something like that.
Doesn't really matter. (!!) What matters is that all four of your rods must be of identical length. That's what matters. Long or short, you can deal with that but only if they are all the same.
But they won't be. There will be some variation in their center-to-center length, center of mass and over-all mass. You'll take care of the weigh differences during balancing but right now you need to know the variation in their center-to-center length, which is pretty easy to measure even with simple tools if you use one journal of a crankcase as your center on the big end and a well fitted wrist pin on the other.
Con-rods are numbered. Use their number in your records when you record the difference in their lengths. SOP is to identify the shortest rod then simply record the differences of the other three as 'pluses.' Good rods, you'll be working in tenths... +8, +4 (ie, +0.0008, +0.0004, etc.)
What's a well fitted wrist pin? Oiled and at room temperature, you should be able to slide the pin into the little-end with your hands. Once in, it should fit well enough so that the pin takes at least two or three seconds to slide out when the rod is held horizontally (and the pin is installed flush). Slower is better. At running temps the rod will expand more than the pin so a good fit is one that is damned tight at room temperature.
A lot of rods aimed at the kiddie trade or used by lo-buck rebuilders aren't even overhauled. They only knurl the bushing then hone it back to size and simply hit the big-end with a hammer before honing, if they bother to hone it at all.
Shop by price, you'll end up buying junk. Good shops are proud of the quality of their work, offer no objection if you want to mike a part now & then. Ditto for good dealers. The other kind don't want anything to do with real mechanics. And get their wish :-)
The point here is that the length of the connecting rods effects the Compression Ratio.
PISTONS & CYLINDERS
Pistons & cylinders are manufactured individually then sorted according to their finished diameter (for jugs) and weight (for pistons). The different sizes and weights are identified by dots of colored paint on the pistons.
In manufacturing a cylinder barrel the raw casting is first machined then the machined barrel is honed to remove the tool marks. In the process of machining a given number of cylinders, the finished bore will become gradually smaller as the tool-bit wears down. When it gets to a certain minimum size they stop the machine and set it back up with a new boring tool. That means the inside diameter of the jugs will fall across a certain range of diameters. This is normal.
The honed jugs are measured and divided into groups according to some standard deviation in their diameter, typically about a thousandth of an inch. But even with that small a standard, with four jugs from the same size-group you can expect to find a variation in their diameter. It won't be much but you need check it.
Volkswagen used cast aluminum pistons from permanent molds. The density of cast aluminum varies slightly according to how much metal is in the smelting pot, its temperature and how long its been there. The castings are then machined to a given diameter, for the grooves where piston rings, for the wrist pin and for the top of the piston. All other surfaces are usually left as-cast. As with all machining operations, the finished dimensions will fall across a range of sizes.
The combination of differing density in the aluminum alloy and variations in the as-cast dimensions causes VW pistons to vary in weight by as much as an ounce (!) Even by 1930's standards that's a bit much so the pistons get sorted into three weight groups with each group having a maximum variation of ten grams.
The nominal dimension of the piston (i.e., its size group) is stamped on the top and a dot of colored paint is used to indicate which direction its actual dimension deviates from the stamped figure. Another dot of different colored paint is used to indicate the piston's weight group and a plus or minus symbol is stamped into the top of the piston to indicate if the piston's weight is above or below the nominal weight for that group.
The pistons are divided into groups according to their weight and within each weight group, are divided into groups according to their diameter, allowing them to be matched with suitable jugs, fitted with rings and packaged for shipment. Stock jugs used to be available individually; nowadays all you'll see are sets of four.
But your carton of new pistons & cylinders may arrive as a grossly mis-matched set of junk. Here's why: Some after-market retailers -- or the clerks who work for them -- tear open the boxes and shuffle sets around to make up sets having the largest bore diameter and identical weight markings. Some dealers even brag about this in their advertising, referring to such sets as the ‘pick of the litter' that need no further balancing. And sell such sets at inflated prices.
It's all bullshit of course, a minor deception aimed squarely at the Kiddie Trade. Why? Because with a weight group encompassing ten grams, with two divisions and a mark for high or low the best you can hope for is a spread of 2.5g... about 25x worse than a real balancing job. (Using an inexpensive electronic scale for measuring and a Dremel tool for removing metal, the average novice has no trouble matching four pistons to within a gram or two.)
But the most interesting point of all this is what happens after those sets of pistons have been pawed over by the clerks. They get tossed back into the boxes willy-nilly and sold to unsuspecting suckers, including other retailers.
The tricky bit here is that you can't balance a set of pistons if they span two weight groups. Pistons are provided with extra metal in the form of ‘balancing pads,' areas from which you may remove metal without effecting the strength of the piston. But the maximum amount you can remove is only a few grams. That isn't a problem when all of the pistons are from the same weight group. But when your P&C's are a mix of two (or more!) weight groups you're liable to see as much as 20 grams difference across your four 'brand new' jugs. Not only does that violate the factory spec of 10g, the difference is too large to be balanced out - - there simply isn't enough metal that can be safely removed.
You just paid good money for a set of new jugs that are junk.
But this is about compression ratio so let's get back to that.
First thing you gotta do is examine your new set of P&C's to make sure they are of the same size group (ie, the variation of diameter) and within the same weight group. That is, all four of the jugs in the box should have the same color code for dimension and the same color basic color code for weight group. The code for plus & minus doesn't matter because you're going to have them re-balanced to a finer standard of precision (i.e., typically +/- 0.1g across a set of 4).
You should do all that before you buy them. And yes, you can get royally screwed when buying through the mail. No, I won't recommend anyone -- I've been sued both ways on that one, once because a guy was unhappy with someone I recommended and another time by a dealer because I didn't recommend him. So go fish. And good luck. Because getting a set of P&C's that hasn't been tampered with is just the start of the story.
Once you have a set of P&C you'll need to put identifying marks on the jugs and record the marks and the dimensions in your notes. I file notches in the flat area of the upper-most fin. When you have more than one engine in the shop at a time, keeping their parts separate can be a problem. I use a series of adjoining notches to identify the set then one to four additional notches, spaced apart, to identify a particular jug within a set. The notches are cut with die-grinder as soon as I open the box. The pistons have to stay with their particular jug so you need to put a matching mark or number on the underside of that piston. I use a vibrating scriber.
Begin your measurements with the distance between the deck lip and the top of the cylinder barrel. The easy way to do this is to just stand the thing on its head and use a surface gauge to find the tallest barrel then record any difference in the other three. Here again, you can expect some small variation.
Barrel length is an especially critical dimension in an horizontally opposed engine since it is the foundation of the valve train geometry. This dimension is even more important in horizontally opposed engines like the Volkswagen which depend upon head studs (or stays) to maintain the seal between the cylinder and the head since any difference in the length of the barrels will impose an asymmetric load on the sealing surface leading to compression leaks.
After measuring the length of the barrels the pistons are removed and the pin height is measured. Follow the same general procedure; put the piston, head down, on a surface plate, use a gauge to find the tallest then record the difference between it and the other. (As a point of interest, in most cases there's nothing to record - - the dimensions match to within less than a thousandth of an inch and an amount that small is generally not significant. What I'm really looking for here is any radical departure from the norm.) But the fact remains, any dimensional variation in bore diameter, barrel length and piston pin height will have some effect on your Compression Ratio
The rings get removed and bunch of other work gets done but we're only talking CR here so I won't go into the other stuff.
CYLINDER HEADS
As with the jugs, when measuring the heads you must first identify them. Through the course of assembling an engine the heads get a lot of work done to them and you need to keep good records. I stamp numbers on them, over by the right-hand exhaust stack (right-hand looking into the chambers, push-rods down). Doesn't really matter how you identify them just so you do. I use stamped numbers because in prepping a set of heads I usually replace some of the guides, run them through the blasting cabinet to roughen up certain areas then open up the chambers, unshroud the valves and do a few other things, most of which will destroy any kind of temporary markings.
On the chamber-side of the head casting you will find either a fully machined flat area surrounding the chambers (old style heads and some after-market types) or six machined bosses, three to each chamber. The horizontal plane defined by the machined surface, either of the bosses or of the flat area, is the base-line for all of your head dimensions.
You need to know the distance from that horizontal plane to the sealing surface of the combustion chamber. More specifically, you want that distance to be as close to identical as possible for both heads and, within a head, for both chambers.
This dimension can be all over the map if the heads have been opened up by a schlock shop. Good shop, any variation should only be a few tenths (ie, ten-thousandths of an inch) up to a max of half a thou (ie, fifty ten-thousandths). Shlock shop, using a cutter in a drill press, you won't believe the crap they turn out.
This dimension is especially critical in the fabrication of a good VW engine. If this distance varies by more than two thou between the chambers of the same head, or by five thou between a pair of heads, have the heads fly-cut by the minimum amount needed to arrive at a uniform figure for all four chambers.
With measurements for the case deck height, barrel length, rod length and piston head height, and knowing the compression ratio you are planning to use, measuring your chamber volumes tells you how much you will have to open them up to achieve the desired compression ratio. Indeed, once you've nailed down a few dimensions, setting up the correct compression ratio becomes something of a no-brainer.
And somewhere about now you'll realize this message wasn't about compression ratio at all :-)
There are two main reasons for doing the work described above. The first is to be able to identify good parts from bad parts. You can't make this determination by price nor the fact the part is new, rebuilt or whatever. Nowadays there is so much junk out there the wiser course is to assume you're dealing with shoddy goods until its specs prove otherwise.
As you progress through the measurement of the parts you begin to see ways in which you can combine those parts so as to arrive at the most dimensionally-uniform result. For example, a slightly short throw on the crank can be combined with a slightly long rod. The same is true for the jugs and the heads in that some combinations may be used to cancel out dimensional variations.
A nice point to keep in mind here is that the ‘assembly' of a ‘paper' engine is an arm-chair activity. You may take as long you wish, shuffling the numbers about in every possible combination until you arrive the one that makes the best possible use of that particular set of parts.
OF THAT PARTICULAR SET OF PARTS...
Did the light-bulb come on over your head? You see, the typical engine-builder can only afford one set of parts. And as much as I hate to say it, if you simply bolt them together the odds of getting a good engine are vanishingly small. Oh, it'll run. Veedubs are robust little buggers... almost anything will run. But if you simply throw the thing together it will not run as well as it should nor last as long as it could. And you won't know the difference.
But I'm not a machinist... (I heard someone shout).
Neither was W. Edwards Deming. He was a statistician with the Bureau of the Census. (Never heard of him? Your loss.)
The truth is, you don't need to be a machinist to build a better engine. You can do that by simply taking a few measurements and keeping good notes. That's enough to keep you from building a total piece of shit. When you subtract the POS Probability Factor from the engine building equation you automatically end up with a better engine. How much better? On average, about twice as good. Yeah, I know... nobody else believes it either. Except for the guys who have done it. (Didja read my article on dialing in your cam? Ditto.)
Up to you. It's your engine.
-Bob Hoover
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