Saturday, January 6, 2007
Dialing in Your Cam
DIALING IN YOUR CAM
You’ve worked all summer to get the bread to build a bitchin 1776 engine. It’s got big jugs, dual carbs, a hot cam and valves the size of dinner plates. You and your buds pull an all-nighter to get it buttoned up in time for school. It fires right up with a lopey idle that sounds way kewl. But punch it, it’s a total POS. Nobody knows why. The timing is dead on and the dizzy checked out. Ditto for the carbs. All your buds agree it should run good but it don’t. A call to the local guru is no help, ‘Bring it in, lemme lookatit.’ At a hundred bucks a glance. But at least it runs, sorta, so you drive it. Maybe it will heal or something.
First day of school Mrs. Wilson who teaches Home Ec and drives that bone stock ‘67 she’s had since high school blows you off pulling out of the parking lot. She wasn’t even trying to dust you. But she did. Bad. The real killer is that your buds saw it happen.
Before you buy a Toyota or transfer to another school let me ask you a couple of questions. Did you dial in your cam? Did you set up your valve train geometry? Have you got any idea in the blue eyed world what I’m even talking about?
“Actually, I’m more into computers ...”
Okay, then think of your cam as the engine’s BIOS. It tells your valves when to open, how far and for how long. The crankshaft is the Master Clock, with Top Dead Center of #1 cylinder as the zero point. Dialing in your cam loads the program at the right address. With the engine above, put your foot down, it should take off like Mad Max blowing nitrous. But only if the crank and cam are in sync.
So didja? Did you dial in your cam? Because even in a stock engine, stack-up errors can put your cam timing out by as much as 4.5 degrees, more than enough to turn your tiger into a turkey.
The Volkswagen was the world’s second economy car ( first was the Ford Model ‘T’). Its low cost of production is reflected by the spec of its parts, which are pretty loose. Some engines came out of the factory sorta sloppy and some sorta tight but the wide tolerances guarantee almost any engine would run. That’s why you don’t dial in the cam doing a rebuild. If you don’t change the cam gear, odds are the rebuilt will run about as well as the original. But when you use non-stock parts, or even a high percentage of rebuilt parts, the odds run the other way.
When you build a high performance engine from a collection of after-market parts, for the duration of the job you better not be into anything but engines. You’re the Mechanic-in-Charge. The chore of making sure things fit falls on you. And one of those chores is dialing in your cam. So didja?
No, don’t tell me. Mrs. Wilson already did.
GAUGING THE CRANKCASE
Building a good engine starts with the crankcase, each of which is just a tiny bit different from every other because of normal variations in tooling wear and production tolerances. The differences are tiny but they’re important. Ignore them and it’s like building a house on a foundation that’s off level by just a tiny bit. The higher you go, the worse it gets. By the time you put the roof on, the thing is leaning like a drunk. One of those tiny differences is the distance between the centerline of the crankshaft and the centerline of the camshaft. It’s not a bunch but if you ignore it, by the time you get out to the valves your high performance engine isn’t.
Because of that difference Volkswagen used nine sizes of cam gear, from +4, through 0, to -4. (The size is stamped on the back of the gear. It reflects a change of .01mm on the diametrical pitch.) About 95% of factory-built engines use cam gears near the zero size, with a nominal range of about +2 to -2. Align boring, which Volkswagen used to do on all their rebuilt engines, dictates the need for the other sizes. How well the gear fits determines how rapidly it wears. Good fit, slow wear. Good fit also means good performance since the fit effects your cam timing and valve train geometry. So that’s where you begin.
Immediately after checking the fit of the main bearings to the crankshaft and case, the driver gear is installed on the crank and the crankcase is gauged to discover what size cam gear is needed. One of the most practical ways to do this to is to obtain three stock cams for use as gauges. With a +2, a 0 and a -2, it takes only a few minutes to figure out the right size cam gear for any crankcase. All you have to do is install your gauge-cams in your crankcase and check their lash against your crank. Here’s how to do it.
CHECK YOUR GEAR LASH
Install the crankshaft into the crankcase half, take your cam and roll the gears into mesh. Don’t worry about the dots, you’re just checking the lash, not assembling an engine.
With the crankcase open, use a pulley or crank on the nose of the crankshaft to smoothly rotate the crankshaft in its normal direction (ie, clockwise when facing the pulley). Do not allow any axial motion of the crankshaft during this test. Using a thrust hub is a good idea. Do several revolutions to insure the two shafts are properly bedded and the gears fully meshed. This test is normally done early in the assembly of the engine, before the connecting rods are mated to the crank.
Spec for cam gear mesh is .000" to .002.” The zero clearance reflects the fact that thermal expansion causes the two shafts to move farther apart at operating temperatures.
To check the mesh, hold the crankshaft stationary, rest your palm on the cam gear and rock it gently back & forth. One of three things is going to happen. You may not feel any motion at all, as if it were bedded in concrete. Or you’ll feel a little motion, usually accompanied by a faint clink-clink as you rock it back & forth. Or you’re going to feel a lot of motion, along with a loud CLUNK-CLUNK .
CLUNK is bad. You’re using the wrong size cam gear; its got too much clearance. Don’t take my word for it, check it. Set up your dial indicator to rest on the bottom-most tooth, right next to the parting line of the crank case. Use a pointed pallet on your dial indicator and set it up to bear on the corner of the gear tooth. Now rock the cam gear back & forth like you did before. (Remember to keep the crank from moving.) If you see more than two thou of movement, you need a larger gear.
If you felt some motion but not enough to give you a clink, the lash is probably okay. But it’s smart to check it out. Set up a dial indicator and measure the lash.
If you didn’t feel any motion the lash may be okay. Or it may be too tight. Try rotating the crankshaft backwards. Here again, do not allow any axial motion of the crankshaft during this test – keep it pressed firmly against the thrust face of the #1 bearing. The handy way to do this is to make yourself a thrust hub. That’s a fancy name for a junked flywheel, cut down to about 6". [See Tools You Can Make] The unhandy way is to use hand pressure. Of course, when you get to dialing in the cam you’ll need to grow a third hand.
If reversing the crankshaft lifts the cam out of its bearings, the gears are jamming, the mesh is too tight. You need a smaller cam gear. But if it rotates smoothly and the cam stays in its bearings, you’re okay.
When gauging your case you start with a 0 (zero) gear. Too tight? Then try your -2. If that’s too loose, you need a -1. Too tight, you need a -3. (Only 13 engines out of 10,000 use a -4.)
The same procedure works the other way. If the 0 is too lose you go up two sizes.
In fully half the engines you’ll build it takes only two trials to nail down which size you need. That’s because over 95% of all VW engines use a cam gear between a +2 and a -2.. A majority of new cases, about 65%, use a +1, 0, or -1. After being align-bored a case may need to go up one size but a +3 case is uncommon, +4 rarer than lips on a chicken. There’s no mystery to any of this, it’s simple statistics.
FINDING A GEAR THAT FITS
For the engine builder without a drawer full of spare cams, finding a gear that fits can be a conundrum. Here’s why. Let’s assume you have a gear on your cam. You check the lash using the procedure above and discover you’ve got either too much clearance or not enough. You need to buy a new gear. But with only one gears-worth of data, you can’t say which size you need. Like all conundrums the gear size question has no satisfactory answer.
What to do? Best bet is to get your case gauged by somebody who knows what they’re doing. If there’s a good VW engine man in your area and you show up with a clean case, the driver on the crank and all the bearings properly fitted, he may be willing to gauge the crankcase for you. It can be done on the bench; no need for the engine fixture.. And it doesn’t take long, if he’s got the right tools, or even if he’s got known-good stock gears to use as gauges. But if you show up with just a box of unblueprinted parts, forget it. There’s at least a couple hours labor to get a batch of raw parts to the point where you can accurately gauge the fit of the cam gear.
If you can’t gauge your crankcase, get your hands on any cam gear of known size. Do the lash check and use your one cam’s-worth of data to approximate a better fit. Follow me through, here. This isn’t as crazy as it sounds.
Cam gear size reflects a Gaussian distribution curve. Statistically that means 95.44% of all stock engines used a cam gear between +2 and -2. We also know that a little bit too much clearance is better than not enough. Armed with that information, let’s play the odds.
Let’s say the gear you have jams. That sez you need a smaller gear. Read the number on the gear. If it is +2, +3 or +4, find yourself a 0 (zero). If it is a +1 or 0 (zero), go find a -1. If it is a -1 or -2, go find either a -3 or -4. If it is a -3, ignore it; the mismatch should be no more than 1/100mm. If your cam gear is a -4 and it is jammed, your crankcase has been improperly align bored because there isn’t any more sizes left. ( I’ve heard there are actually eleven sizes of cam gear, +5 through -5, but I’ve never seen a 5 and can’t say they actually exist.)
Now let’s look at the possibility your cam gear has too much lash. This is an easier problem to solve because we have more data. The fact it is too loose tells us we need a larger gear. By measuring the amount of that looseness – the excess lash – we can estimate what size will be a better fit.
First off, expand your acceptance spec to .004". If your gear measures less than .004" lash, go ahead and use it. It’s sloppy but it’ll run. If it measures .004 to .008, go up two sizes. If more than .008, go up three sizes.
You can’t build a good engine with guess work but an educated guess, making full use of what information you have is better than pretending your gear lash doesn’t matter.
When you know what size gear you need, give Clyde Berg a call, see if he can help you out. His dad used to keep a pretty good stock of different size gears on hand for his cam customers. Or do like I do and head for the junkyard. Because the cam runs at half the speed of the crank, the cam may be junk but the gear is usually well within spec. Simply drill out the rivets, throw away the cam and you’ve got yourself a usable gear.
HOW NOT TO SAVE MONEY
To keep down the cost a lot of guys use a reground cam. Not one of the good ones, the other kind, with the gear already attached. You know the ones I mean, you’ve seen them at swap meets and in the J. C. Whitney catalog.
Such a cam is not a good choice because its gear is probably the wrong size for your engine.
Since you may need any one of nine different gears, most cam grinders ship their wiggle sticks without any gear at all. The flange is drilled & tapped (usually for M8x1.0) to accept cap screws. As the Mechanic-in-Charge it’s your responsibility to install the proper gear. But nowadays the trick is finding the proper gear.
ONE SIZE FITS ALL – NOT!
So you buy a hot cam for your dream machine. Now you need a cam gear. You drop by the local VW store and sure enough, there’s a batch of cam gears hanging on the wall.
Odds are, they won’t fit either.
In preparing this article I examined more than thirty after-market gears obtained from a number of retailers here in southern California. Most of the gears were from Taiwan, some from Germany. All of the after-market cam gears I examined were not marked as to size. Of the Taiwanese gears I checked, all were about a +3. This may be an example of Oriental humor since a +3 is too big for 98% of all crankcases. ( To find out what size they are you have to set them up in an engine and compare their lash to gears of known size or measure their diametrical pitch. But unless you want to tool up for it and do them in batches, it’s impossible to justify the time it takes to determine the size of an unmarked gear, so long as gears of known size, even used ones, are available at junkyard prices. )
The people selling those oversized, unmarked gears worked pretty hard to convince me gear lash is no big deal. I was told that after-market gears only come in one size because it’s made of cast aluminum, much softer stuff than the magnesium alloy used for stock gears, and will wear itself in.
I got the same story at different places, often delivered in a scornful tone of voice. Oversize gears wear themselves in. Everyone knows that. So what about undersize gears? They don’t matter, according to a clerk about twenty years old who claimed to have run one for the last five years (!) in his 250 hp daily driver. Gives him more power, he sez.
Sure it does. (Can I get fries with that?)
Allow me to offer a bit of advice based on more than forty years of VW engine building experience. What an oversized gear does is wear itself out and quickly, too, along with your engine. The first time you fire it up, jammed against the steel driver gear the softer aluminum wears at a furious rate, generating spoonfuls of metal flakes to contaminate your bearings. That’s where most of that non-magnetic metallic sludge comes from in lo-buck rebuilds. By the time the engine reaches its normal operating temperature and thermal expansion draws the gears apart, it’s too late, the thing will be worn beyond spec.
You’re the Mechanic-in-Charge. Deciding which gear to use is up to you. But before you buy in to the one-size-fits-all philosophy, keep in mind that philosophy is saying Volkswagen, with nine sizes of cam gear in more than twenty-two million engines was wrong. Personally, I found most after-market cam gears to be shoddy goods due to their poor fit with the camshaft flange. Notable exceptions were gears of German manufacture which usually come drilled only for 6mm rivets. (Old stuff. Box said ‘W. Germany.’) They were a uniformly tight fit on the flange of the camshaft and although unmarked, ran about +1 in size. Opening up and counterboring the rivet holes to accept cap screws is a simple task.
If gauging the case says that a +3 is just the size you need, one of those cast aluminum jobbies from Taiwan may be justified. But a word of caution: Take your cam with you and do a trial fit before you buy. The flange of the camshaft must fit tightly into the socket on the cam gear. The fit of the flange to the socket is what provides axial alignment between the camshaft and its gear. Although it’s rather hard to believe, most of the after-market gears I’ve examined simply did not fit, the spigot was too large, too small or the bolting holes were misaligned.
Once your cam has been blessed with a gear that fits we can determine the indexing error between the cam and the crank. To do that we’ll do a partial assembly of the engine and set up our degree wheel. But first we need to find TDC.
WHERE IS TDC?
One Tuesday afternoon in 1873 Nicholas Otto invented the four stroke engine. On Thursday he dropped his TDC, it rolled under a bench and got lost. Mechanics have been looking for TDC ever since.
All piston engines have a TDC but there’s two Top Dead Centers in the Otto cycle and two ways of defining it. The TDC we’re interested in is the one on the compression stroke (the other is on the exhaust stroke). When setting the compression ratio or adjusting volumetric balance we define TDC in terms of deck height. But for cam timing, TDC is defined in terms of crankshaft rotation. Any way you cut it, before you dial in your cam you gotta find your TDC. So let’s do that.
At this point I’ll assume you’re using a cam gear having the proper mesh, the cam bearings are fully bedded in their saddles and the cam’s end-float is within spec. I’ll also assume you’ve set your crankshaft end-play and are using a thrust hub.
In the following procedure when I mention rotating the crankshaft, always turn it in the normal (ie, clockwise) direction unless told to do otherwise. The angle of the cam gear teeth combined with the end-float of the two shafts generates a surprising amount of slop any time the direction of rotation is reversed. When you need to back up and try again, always go back at least a quarter turn of the crankshaft. This insures you’ve taken up the slop. When asked to rotate the crankshaft to a specific point, do so with a smooth continuous motion. Don’t jiggle the thing back & forth. Jiggling about introduces slop into your readings and you’ll never get the same numbers twice in a row.
Install a pair of modified cam followers on the #1 cylinder (See Tools You Can Make). Install the #1 connecting rod and torque the cap to spec. Close the case, install the six large (M12) nuts with washers and torque to 24 ft-lbs in a ‘W’ pattern, checking for free rotation of the crankshaft as the torquing progresses. When torqued to spec the crank should turn freely with finger pressure. Install the #1 piston, without rings, onto the #1 con rod. Inspect the cylinder sealing surface on the case and cylinders then install the head studs. Install #1 & #2 cylinder barrels with their spacers, if any. Install the deck plate, washers, spacers and nuts (See Tools You Can Make) then torque to spec for your particular studs (ie, M8 = 18 ft-lb, M10 = 30 ft-lb).
TDC - DIAL INDICATOR METHOD
Once the deck plate is in position you may install your dial indicator, timing wheel and timing wheel pointer. (See Tools You Can Make.)
The markings on your timing wheel will put you in the vicinity of TDC and your dial indicator will tell you when you’ve arrived. Use the lifters to identify which TDC you’re on. At TDC on the compression stroke both of your valves will be closed, meaning the lifters will be down. Rock the crank through at least ninety degrees of arc at least half a dozen times to confirm the reading of your dial indicator. Once you’re satisfied you’ve found TDC, position the degree wheel pointer precisely upon the TDC mark.
TDC - STOP-BOLT METHOD
Finding TDC with a dial indicator works fine with most engines based on VW components but as the stroke increases so too does piston dwell at the point of reversal. If you’ve got good equipment – and young eyes – the dial indicator method will work for any engine although the probable error will increase with the stroke.
The stop-bolt method of determining TDC eliminates the dial indicator and any dwell-induced error. You insert a bolt in the torque plate so as to stop the upward travel of the piston before it reaches TDC. You put a piece of masking tape or a white stickum on your degree wheel centered on the as-marked TDC and extending to either side. Rotate the crank until the piston is stopped by the bolt. Do this gently so as not to mar the top of the piston. Keeping tension on the crank, make a mark on the tape precisely aligned with whatever pointer you’ve rigged. Reverse the rotation of the crank until the piston again is stopped by the bolt. Make a second mark on the tape, again precisely aligned with your pointer.
TDC is exactly half-way between the two marks.
A lot of guys go astray by trying to use the stop-bolt method without marking their degree wheel. Instead, they record the stopped position in degrees, such as -3 going one direction and +4 going the other. Then they go crazy and subtract three from four and declare TDC to be at the +1 degree mark. Which is close but not nearly close enough. The correct answer is the difference divided by two, or half a degree.
To keep from going crazy, when using the stop-bolt method ignore the degree wheel markings. Make your own marks, measure the distance between them and divide it by two. The result is TDC with an accuracy of about half a degree.
INDEXING THE DEGREE WHEEL
The whole idea behind finding TDC is to index our degree wheel. The reason we need to index the wheel is because every engine is slightly different. When dialing in the cam we find TDC with as much precision as possible, move our pointer to align with the degree wheel’s TDC mark and tighten it down. Some guys get confused on this issue because they think cam timing and ignition timing are the same thing or that TDC is represented by the centerline of the crankcase. The centerline is just a handy reference used in conjunction with the stock pulley to locate the approximate position of the static timing point for the ignition system. Ignition timing is akin to horseshoes, where close enough is usually good enough. Cam timing is the fixed relationship between the cam and the crank. With cam timing, you’ve either got it right or you lose.
Perhaps it would help resolve the parting line confusion if we started with a degree wheel that had no marks at all and covered up the parting line with tape. When the piston is at TDC so too is the degree wheel. You may position your pointer anywhere on the edge of the pulley, pencil in a mark and call it TDC. Or you may chose to put the degree marks on the engine and place the pointer on the wheel, which is what Volkswagen did with the Type IV engine.
HOW TO SPEAK CAMLOBIAN
To dial in your cam you have to be able to read its specs; to understand a cam card. To do that you need to speak Camlobian.
At first glance Camlobian seems crazier than a hoot owl in heat. It is not. What’s crazy is the description of the Otto cycle as taught in Auto Shop 101, where the two revolutions of the crankshaft are neatly divided into four distinct strokes during which the valves pop open and snap closed precisely (and instantly) at TDC and BDC. Real engines don’t work like that. And never did. Those pretty pictures in all those text books are as phoney as Washington chopping down the cherry tree. Or cam gears that wear themselves in.
The fuel/air charge has mass and mass has inertia, as do all components in your valve train. It takes time and energy to overcome inertia. You must initiate the opening of a valve well before such opening is needed and start closing them well ahead of when it must be fully closed. That’s why the intake valve in a real engine starts to open during the exhaust stroke and the exhaust valve opens rather early on in the power stroke. At one point, the two valves are even open at the same time.
Camlobian reflects the reality of Otto cycle engines by ignoring the four strokes and focusing on intake and exhaust events. It does this by combining the 2:1 relationship between the crank and cam into quadrants of crankshaft rotation during which particular intake and exhaust events normally occur. The quadrants are identified relative to Before (B) and After (A) Top Dead Center (TDC) or Bottom Dead Center (BDC) and are named according; BTDC, ATDC, BBDC and ABDC. (Not to worry. I put all this poop on the degree wheel I’ve included with this article.)
What this form of notation does is convert each cam event into a logical data set, unique from every other. For example, the intake valve opens (IO) in the BTDC quadrant of the exhaust stroke. Since each quadrant represents 180° of cam rotation, so long as we’re speaking of automotive Otto cycle engines a particular event will always occur in its particular quadrant. Having opened in the BTDC quadrant the intake valve must close (IC) at some point during the ABDC quadrant of the compression stroke. In a similar fashion, the exhaust opens (EO) during the BBDC quadrant of the power stroke which means it must close (EC) in the ATDC quadrant of the intake stroke. (And yes, there are some exceptions. Most occur with cams for supercharged engines, where you will occasionally see a quadrant number larger than 90 or less than zero. The basic definitions remain unchanged.)
Now comes the neat part.
Having established those conventions, speakers of camlobian needn’t bother to mention either quadrant or stroke. ‘The intake valves opens 18 degrees before Top Dead Center on the Exhaust Stroke‘ becomes simply ‘IO 18.’ Some cam cards are even more terse, such as ‘I 18-50, E 14-54,’ wherein 18 is the opening point, 50 the close. Since by convention the intake valve is listed first, a cam’s timing may even be defined by the ultra cryptic ‘18-50/14-54.’
Although Camlobian is a culturally rich tongue I’ve cited only a few basic phrases, enough for you to understand a cam card. For dialing in a cam, for each lobe, we’re only interested in three of its many events. We want to know when it opens (O), when it closes (C) and when it peaks (P). To adjust our valve train geometry, normally done in conjunction with dialing in the cam, we also need the maximum lift and the half-height point but that will have to wait for another article. And perhaps another language.
.050" SPECS vs ADVERTISED SPECS
Because there are several ways to grind a cam and some marvelously ingenious methods of selling them, cam grinders have agreed to use the 0.050" lift point as a common standard for determining when various events occur. This is called the checking clearance.
You must read cam ads very carefully. The advertised specifications often use something other than the .050 checking clearance as their base point and may even refer to valve lift rather than cam lift, coyly neglecting to define rocker ratio. Such deceptive practices are used by some folks to sell stock sticks as full-race flame throwers.
CLOCKING THE CAM
For obvious reasons a dial indicator is commonly called a clock. Or perhaps not so obvious in these digital days. (Early clocks only had one hand.) To a machinist, automotive or otherwise, a clock is a dial indicator. To clock the cam means to measure it’s lift, and determine its timing relative to the crankshaft.
The trail of tasks which have lead you to his point -- adjusting the lash of the cam gear, finding TDC and indexing your degree wheel -- have laid the foundation for the accuracy of the measurements you are about to make. If you think you could have done any of the preceding tasks better, go back and do them over because dialing in your cam is a classic case of GIGO – the output will reflect any inaccuracies in the input.
Your cam should have come with a data sheet, probably in Camlobian, listing when the valves open and close. Clocking the cam will tell us when these events occur in this particular engine. It’s important to understand that normal dimensional variations in the manufacturing process combined with your method of measurement and the imprecision of your tools guarantees you will see some error in the cam’s timing. Your purpose is to find out how much. If it’s less than one degree you may decide to accept it. If it is more than five degrees at the crank you might want to try another cam. But in a majority of cases the error will be a couple of degrees, plus or minus, and you will elect to reduce it as much as possible by adjusting the relationship of the cam to the crank using one of the methods I’ll describe in a minute.
By convention we do the intake first so begin by setting up your dial indicator to read off the modified tappet installed on #1 cylinder. (See Tools You Can Make for a holding fixture.) In all cases, on the VW engine the intake valves are those in the middle of the engine; the exhaust valves are the ones on the corners.
Slowly rotate the crankshaft through several revolutions while watching the dial indicator. You will see a prolonged period where the indicator makes no movement then rises, rather rapidly, to some peak value before dropping back. The prolonged period of no movement is when the tappet is riding on the heel of the cam. We need to find the middle of the heel. To do so, note when the peak reading occurs and mark that point on your degree wheel. Rotating the crankshaft one complete turn from the peak should put you in the middle of the heel for that lobe. Zero your clock at that point. (Simply loosen the lock and rotate the dial until the zero-mark is aligned with the needle then re-tighten the lock. If your indicator is properly mounted it will remain steady as a rock while being zero’d. If not, it needs a better mount.)
Once the indicator has been zeroed it may be used as a measuring device. Slowly rotate the crankshaft to measure maximum cam lift. Count the turns or use the turn indicator to keep track, remembering that the first revolution is the ‘zero’ turn. That is, if the needle passes through zero four times before coming to rest on 29 the dimension measured is .329" Record both the lift and the timing.
Once you’ve zero’d on the heel and found max lift, return to the middle of the heel, rotate the crankshaft until the cam follower has risen exactly .050". Record the reading from the degree wheel as IO (ie, Intake Opens) and reset the dial indicator to zero at this point. This is the .050" checking clearance point. Once your clock is zero’d, rotate the crank in the normal direction until you return to zero. Record the reading from the degree wheel at this point as IC (ie, Intake Closes). (By convention, I’ve used .050 for the checking clearance. Use whatever checking clearance is specified for your cam. Cams from metric countries typically use 1mm (~0.040"). )
With the indicator zero still set at the .050 checking point, find the peak lift and record it as IP (ie, Intake Peak). It should occur at the same point as before but the max lift will be less because we’ve reset our dial indicator to zero at the .050 lift point..
Divide the Intake Peak reading just recorded by two. This is the 50% Lift Point. Write it down. Now go find it! Go back to zero and rotate the crank until the clock reads 50% of max lift. Record when this occurs by reading the degree wheel. We’ll need this information when we adjust your valve train geometry.
Move the dial indicator over to the exhaust tappet for #1 cylinder and repeat the above tests. Record your findings. If you’re using a split lift cam, such as a stock VW stick, be very careful to record the 50% lift point for the exhaust.
WHICH WAY? HOW FAR?
Once you’ve found TDC, indexed your degree wheel and clocked your cam, the data you’ve collected tells you if the cam is properly indexed to the crankshaft. It won’t be. When clocking a cam the question is never if there is any error but how much and in what direction.
Did that come across? The reason we’ve gone through all this is to find out how big an indexing error we’re dealing with. Once you know what the error is, you can decide if it’s significant. As a general rule for street engines, an error of one degree or less, plus or minus, is not considered significant. Unless you like to build really good engines. In which case your standard of excellence will vary from zero error to some fraction of a degree.
When you build your own engine you’re not punching a time clock. There’s no foreman breathing down your neck. You don’t have a ten-engines-a-day nut to crack like the sweat shops cranking out those shoddy lo-buck rebuilts. When you build your own engine there is only you and the tools and the parts. There is absolutely no reason for you to settle for less than the very best you are capable of doing.
PULLING IT ALL TOGETHER
I’m building a low rpm, high torque engine to run on natural gas. After gauging the case, finding a suitable cam gear ( a +1) and modifying it to accept cap screws, I did a pre-assembly and started clocking the cam, a Schneider 248-F. After clocking it a couple of times my notes read:
IO = 2 IC = 38 EO = 38 EC = 2
Unfortunately the cam tag read: ‘4-36, 40-0' Translated, that meant
IO = 4 IC = 36 EO = 40 EC = 0
The numbers say the cam matches its specs, which is good, but they also say there is an indexing error of 2 degrees (retarded) measured at the crankshaft. To dial in the cam it needs one degree of advance.
Did that come across? Your crankshaft rotates twice in the time your camshaft rotates once. When the crankshaft rotates 720 degrees, the cam shaft rotates only 360. Two degrees of rotation at the crankshaft translates into one degree of rotation at the cam shaft.
Once we know how much the cam needs to be adjusted we have to figure out which direction it should be rotated. The gearing between the crankshaft and the cam causes the cam to rotate in opposite directions. Since the crankshaft rotates clockwise, to advance the cam we need to rotate it to the left or anti-clockwise. To retard it we would move it to the right. Always keep in mind that any adjustment is applied to the camshaft and not the gear. The gear remains fixed, relative to the crankshaft.
Once we know how much the cam needs to be rotated and in which direction, we need to know how far that amount of rotation is in dimensional terms. To figure it out we simply have ourselves a piece of pie. Or rather, π. (See the drawing.)
The radius of the bolting circle on the flange of the cam is about an inch and an eighth, something like 2.244" on the diameter. One degree on a diameter of 2.244" is about .019". That tells us how far we need to rotate the cam, which is fine if you happen to be a cap screw. For humans, a handier measure is to use the outer diameter of the cam’s bolting flange, the thing that sockets into the recess on the back of the cam gear. One degree on the diameter of the flange is about .024.” If we scribe a line across the cam’s flange and the gear, we can gauge degrees of rotation by measuring the displacement between the scribed lines and dividing by 0.024."
DIALING IN YOUR CAM
The term dialing in the cam comes from watching the needle of your dial indicator ooze toward zero as you make the adjustment. Given everything you’ve done to arrive at this point, the dialing-in procedure is anticlimactic, a hoo-hum no-brainer. Simply bring the degree wheel to whatever set-point you’re using, loosen the cam gear’s cap screws, lock the crank in position and rotate the cam until your dial indicator reads zero. For example, let’s say we’re using IO as our set point. Our clock has been zeroed at the .050 checking clearance. With the degree wheel set to IO, the dial indicator should read zero. It doesn’t but that’s okay; that’s why we’re here. Simply lock the crankshaft at the set point (IO in this case), loosen the cap screws and rotate the cam while watching the dial. When the needle touches zero the cam is at IO. And so is the crank. And that’s what we want. Tighten down the cap screws and move on to setting up your valve train geometry.
The main objective of this article is this one procedure, so allow me to go over it again. All of your work up to this point has been to cause the position of the needle on the dial indicator to reflect the difference between the indexing of the crank and the indexing of the cam. At this point you don’t care what that difference is, you’ve already measured it and determined it’s within your range of adjustability. The crankshaft is locked in position but the camshaft is not. The dial indicator, which is pointing at a figure other than zero, is riding on the cam. So you reach in through the oil pump hole with a tool and twist the camshaft – in whatever direction – until the needle reads zero.
If your cam was accurately fitted, dial indicator firmly mounted, TDC accurately determined and the degree wheel accurately indexed, zeroing your clock will dial in your cam to better than one-quarter of a degree. No myths, no math, no science and no expensive tools.
What makes this procedure a no-brainer is being able to adjust the cam when it’s inside the crankcase and its gear is locked in mesh with the crankshaft. The ability to do this -- the secret of turning a tough job into a five-minute no-brainer -- depends on two factors. The first is some provision that allows the cam to be rotated relative to its gear without removing the cam from the crankcase. There are a number of ways to accomplish this and I’ve described two of them below. The second factor is that the flange of the cam must be a tight fit in the spigot on the gear. If it’s not, when you loosen the fasteners and rotate the cam, any slop will be transferred to the gear. In effect, you’ve just shoved the gear to one side. The axis of the gear’s rotation is now different from that of the cam. That means the cam gear’s rate of rotation will not be uniform. This leads to a whole shopping list of problems including accelerated wear and poor performance.
In my opinion, the best method of achieving dial-in adjustability is by machining the bolting hole and its counterbore on an arc. (See the drawing.) Since this cuts away a good deal of the cam gear, stepped steel washers are used under the cap screws. The steel washers, commonly called cam buttons are symmetrical. Thanks to the use of cam buttons, this method is strong enough for all but the most powerful engines, plus it offers the convenience of being able to dial in the cam while it’s in the crankcase.
Gene Berg used to sell a good dial-in cam gear. And in any size you needed, so long as it was for one of his cams. If you don’t want to make one up yourself, give Clyde a call, see if he still has some.
If you prefer to roll your own by modifying a stock gear (which is what I do) you’ll probably find the easiest way is to use a rotary table and a milling machine, but other methods will work. I saw a guy in Baja doing a nice job on a cam gear using a router with the cam gear mounted in a wooden fixture. You wouldn’t think it would work but it did a pretty good job. I guess when you don’t have a shop full of tools you have to be a little smarter than the average bear.
THE FAT HOLE METHOD
Another way to achieve dial-in adjustability is by simply starting out with a fat hole for your fasteners. An M8 cap screw has a diameter of only 7.8mm, which means it has .004" of clearance in a 5/16" hole. Open up the hole to 11/32" and you end up with .0358" of clearance for a .308" bolt. You may now adjust the cam by nearly a full degree, plus or minus. That’s enough to reduce a two degree index error at the crank to under half a degree, good enough for most work.
Everyone who understands the need to dial in their cam has used the fat hole method at one time or another. Unfortunately, some engine builders use only this method, opening up the bolt holes to a whopping .375". Used with a small washer, that gives them about sixty-thou of slop, a full +/- three degrees at the crank, enough to dial in almost any cam. But counterboring weakens the cam gear and opening up the bolt hole makes matter worse. The risk here is that, having successfully used the fat hole method to build engines needing only a small amount of adjustment, they eventually try hogging out a huge hole and pushing the cap screw clear over to one side. Now it’s going it fail. And take the engine with it.
NON-DIAL-IN METHODS OF ADJUSTABILITY
The following methods of adjusting the cam gear require removing the cam from the engine to do the adjustment. After adjusting the gear always repeat the clocking procedure. Indeed, when dialing in a cam, regardless of the method used, it’s a good idea to verify the timing. Dialing in a cam is surprisingly easy once you learn how. Dialing it in wrong is even easier and there’s no training required.
CAM BUTTON METHOD
First off, they aren’t buttons they’re stepped steel washers. Eccentric steel washers, in this case. (See the drawings.) How they came to be used is pretty obvious once you’ve dialed in a few cams using the fat hole method. It has to do with the fact that counterboring weakens the cam gear and with how fat a hole can you go. The answer is not fat enough, without causing the gear to fail. But let’s say you hog out a 7/16" hole in the middle of your 3/4" counterbore. To provide support for the cap screw and prevent failure of the cam gear, you make up a stepped washer as shown in the drawing.
If you make up the washer so the pilot – the stepped portion -- is concentric to the bolt hole, your cam will be indexed straight up, without advance or retard. You may then install the cam and clock it. If clocking the cam sez you need to move it two degrees, you go over to the lathe and make up three new buttons with the hole offset by forty thou. That may sound like a major chore but trust me here, making eccentric buttons is a trivial task, assuming you have access to a lathe and know how to twirl the knobs. An 8th grader in metal shop class can crank out half a dozen engine’s worth of cam buttons before the bell rings.
Once you know how much adjustment you need and have the buttons in hand it’s usually quicker to tear down the engine rather than try to work through the oil pump bore. Yeah I know; some guys say it takes them only a few minutes. Your mileage may vary.
FILE TO FIT
Yup. Just like it sez. Start with a stock cam of the correct size, counterbore to 3/4", open it up to 5/16", install on the camshaft, assemble the case and clock the cam. When you know how much and which way it needs to move, tear it down and go at the bolt holes with a chain saw file, moving the hole in the direction you want the cam to move .020" for each degree.
If you don’t have a lathe or a box full of cam buttons, so long as the required adjustment is no more than 4 degrees at the crank, filing the gear to fit is the lo-buck winner. Four degrees at the crank is two at the cam so you move the hole forty-thou; about 1mm. If you go more than forty-thou you’ll have to use a smaller washer under your cap screw and things are liable to break.
Filing to fit isn’t the smartest solution. Buttons are stronger and more accurate. But moving the bolt holes with a file is the cheapest solution and when you’re young you can’t always afford to be smart.
WOODRUFF KEY METHOD
When you’re forced to use a pre-assembled cam/gear combo the use of an offset Woodruff key is your most practical means of making any adjustment to the timing. Volkswagen used to offer offset Woodruff keys as a special order item. They came in about five sizes and cost the same as the straight key, except you had to wait for it.
A big joke back then was to ask a new parts guy for Woodruff with a minus two degree offset and watch him go flipping through his book. This was a real knee slapper, on the same order as a left-handed monkey wrench. (Offset Woodruff keys don’t come as plus or minus... you simply install it with the overhang on the right-hand side of the slot to retard the cam, on the left to advance it. In other words, as with the monkey wrench, you simply turned the thing over.)
Offset keys were catalogued by degrees at the cam which could lead to confusion since American mechanics normally dial in the cam relative to crankshaft degrees. No problem, just divide your crankshaft-based index error by two. Of course, it’s even less of a problem nowadays since such parts are no longer available.
An automotive machinist can make any kind of Woodruff key you want, with any amount of offset up to a maximum of about 10 degrees (ie, an offset approximately half the width of the key). But expect to pay a good price for it. It might cost a bit less if he starts with standard #1210 Woodruff key, the closest match to the metric size used in your engine, but he can only give you about 4 degrees because that’s all the width he has to work with, a #1210 being 3/8" wide. And it’s still going to cost you something because it’s a fairly tricky bit of work to set up. Fortunately, an adjustment range of +/- 4 degrees is usually more than enough to cover the usual range of cam timing.
If you need an offset Woodruff key, give the machinist the driver gear and a new key and tell him how much offset you need in crankshaft degrees. He’ll use the stock parts to figure out the dimensions of the new key. As a point of interest, the bore of the driver gear is about 1.645" which tells you one degree is about .01435" at the Woodruff key (ie, 1.645 times pi, divided by 360 equals one degree). The amount of the overhang is equal to the number of degrees you want to change the timing times .0144". I’ve also included a drawing of the stock Woodruff key but don’t take the dimensions as gospel; measure it for yourself.
AFTER-MARKET ADJUSTABLE CAM GEARS
If you want to try an adjustable after-market cam gear I strongly suggest you keep your money in your jeans until you’ve inspected the part. Take your cam with you and try it in the spigot. You want a good tight fit. Then make sure the buttons fit the counterbore & hole in the cam. Finally, bolt it to your cam to insure the holes are properly aligned.
MAKING IT GOOD
Okay, so you got the cam dialed in to within a gnat’s a**. What’s going to keep it there?
After setting your cam timing, dismantle the engine, remove the cam, put it face down on the bench and make a couple of witness marks where the flange of the camshaft nests into the recess on the back of the cam gear. Make these distinctive from any other marks and make a note of their location in your documentation package. If you need to dismantle the cam from the gear, the marks will insure it goes back together properly.
Examine the cam-gear cap screws. Are they drilled for safety wire? Have you got one of those little drill blocks? Can you even use safety wire on the fasteners? (If you can, you should.)
Remove the cap screws one at a time. Clean them with MEK. Using a Q-tip, clean the threaded bore in the cam shaft. Reassemble using high strength Loctite and lockwashers. Torque to 10 ft-lb. When you’ve cleaned, Loctited and torqued all three, retorque to 14 ft-lbs. If possible, install safety wire.
(Ed.Note: This article was published in 2001 the Nov. and Dec. issues of 'VW Trends' magazine and was supported by about two dozen illustrations.)