Saturday, November 25, 2006

AV - Lapping Jugs, Sealing Rings and Why the Sky is Blue

Recent posts to this Group (Ed.Note: CX4) caused a number of messages to appear in my mailbox. Unfortunately they could not be answered with a simple yes or no and one was specific to the Type IV, on which I'm not qualified to answer. When a fourth message arrived on (generally) the same subject it seemed that a general answer, publicly posted, would be the best solution. So here it is.

To begin at the beginning, Volkswagen did not lap the barrels to the heads but they DID install sealing rings between the barrels and the heads of the Type IV engine.

Why didn't they lap-in their barrels? Because there was no need to do so. The top of the barrel and the sealing surface inside the combustion chamber were both perfectly flat, hence there was no need to lap them in.

So why does everyone think it's a good idea? Well.... `everyone' DOESN'T think so :-)

Volkswagen was the first major auto manufacturer to use gasket-less assembly. That is, surfaces were machined so accurately that it took only a thin wipe of sealing compound, typically Permatex Type 3, to produce a leak-free fit of the crankcase and transmission halves. (Note: Early VW trannys were split down the middle, just like the crankcase.)

To achieve a leak-free fit between the aluminum heads and the cast iron barrels Volkswagen used some very sophisticated engineering. First, they made sure the sealing surface inside the combustion chamber was perfectly flat and that the depth of the sealing surface was PERFECTLY EQUAL in both chambers. Then they used cast iron barrels having a WIDE sealing surface that was also perfectly flat. Smooth, too. (The sealing surface of new 77mm jugs looked like mirrors.) Finally, they provided approximately 170 FT/LBS OF TORQUE to the head-stays.

Which of course is impossible.

The head-stays are merely hand-threaded into the magnesium crankcase for less than an inch. Even with the coarse pitched M10x1.5 thread you'd need nearly twice that depth to withstand 170 ft/lb of torque.

What VW provided was the amount of TENSION approximately equal to that produced by torquing the head-stays to 170 ft/lbs. How they did this is perhaps the trickiest bit of engineering in the whole engine because the assembly-torque was only 23 ft/lb (18ft/lb for the later model 8mm dia studs).

This seemingly impossible bit of magic was accomplished by taking into account the radically different co-efficients of thermal expansion between the magnesium crankcase (in which the head-stays are screwed), the cast iron barrels, and the cast aluminum heads (which are secured to the head-stays with nuts & washers). Here comes the tricky bit: As the cast aluminum heads heat up, they try to expand AWAY from the cooler cast-iron barrels, which have a much lower coefficient of thermal expansion. But the head-stays prevent any motion between the cylinder head and the barrels. This causes the expansion to appear IN the head-stays as TENSION and it is this tension that clamps the heads to the barrels with sufficient force to ensure a leak-free fit even when subjected to the pressure of combustion. And that's why it wasn't necessary to lap-in the barrels. (Notice the past tense? :-)

So why does `everybody' think it's a good idea? The main reason is because THEIR surfaces are NOT flat. Or they may be flat but of unequal depth. Here's why: Parts heat up when they are machined. Volkswagen machined both combustion chambers simultaneously on a superbly rigid machine, taking the thermal growth resulting from the machining operation into account. The end result is heads that are virtually identical, especially with regard to the flatness and depth of the sealing surface.

By comparison, a shade-tree mechanic opens up the combustion chambers ONE AT A TIME using a spindle-type tool bolted to the head and driven by a drill press or even a half-inch drill-motor. After cutting one chamber, the tool is dismounted and re-assembled over the second combustion chamber and the process is repeated. But after cutting the first chamber, unless you wait at least an hour for the head to cool down, the depth of the second chamber is going to vary by a significant amount due to thermal expansion. Which is only part of the problem...

As for the surface finish, the typical spindle-type cutter has only one cutting edge, which must be at least 3/4" long. When opening up the heads to accept 92mm barrels you're looking at a hole nearly four inches in diameter (~3.978", givertake... ideally, the cut should match the diameter of your set of barrels plus about half a thou per inch of diameter [remember, cast iron expands less than aluminum - - the heads are going to expand more than the barrels, hence the relatively tight fit, which guarantees better alignment during a cold start]). The recommended tool-speed for cast aluminum is about 100 surface-feet per minute, which is also about 1200 inches per minute. Since see equals pie dee that means our cutting tool should be rotating at NO MORE than about 100 rpm and in this case slower would be better.

This cutting speed is easy to achieve with a milling machine but impossible with the typical drill press which usually can't go below 300 rpm. (What's the lowest speed on yours? Many drill presses can't go below 500 and the typical half-inch drill-motor spins between 800 and 1200 rpm.)

Wanna know what happens when you try to open up a set of heads with the cutter spinning at 300 rpm? You get a lot of `ripple' - - the cut surface is NOT FLAT, it's sorta wavy. And the faster you go, the worse it gets.

And that's why `everybody' laps in their jugs... because they HAVE to.

On the other side of the coin are guys who use a real milling machine running at mebbe 50 rpm. The head is rigidly secured in a fixture that supports the over-hanging portion of the combustion chamber. The mass of the machine, which is bolted to the concrete floor of the shop, guarantees there is no vibration, whilst the slow spindle speed - - typically 50 to 80 rpm - - reduces the chance of any harmonics to below the level where they can effect the flatness of the finished surface. In addition, the cutting tool is either flooded with coolant or the head is allowed to cool between cuts so that the finished depths will be identical. End result: Perfect flat sealing surfaces of identical depth... that do not need any lapping.

Now back up about a thousand words and note the third reason Volkswagen didn't lap-in their jugs: The jugs had a nice wide sealing surface. Or at least, they did have, up until the 1500 engines :-) That's when VW bored out the stone-reliable 77mm jugs used on the 1200 and 1300 engines to come up with the 83mm jugs used on the 1500. And over-bored the 83's to 85.5mm for the 1600. Which tended to leak like a bitch no matter what you did.

The reason here was pretty simple: They had increased the bore of the cylinder at the same time they'd reduced its sealing surface. (Hang on to this fact. It plays a major role in most flying Volkswagens.)

Volkswagen knew they had a problem with leaky cylinders. As early as 1965 there were plans to replace the Type I engine's 69mm crank with one of 74mm, and go to an 88mm jug having thicker walls. This would have given them an 1800cc `Type I' engine with about the same cylinder sealing surface of the ultra-reliable 1300. Initially it was to be installed in the Type III's but the odds are overwhelming that it would have found its way into all other chassies. Then Heinz Nordhoff died (April of `68), bean-counters gained control of the company and virtually all R&D was abandoned in favor of short-term gains.

`Machine-in' 88's remained available from after-market sources and once their value was realized they were quickly displaced by `slip-in' 88's aimed directly at technologically naive VW owners who didn't know the difference between `slip-ins' and `machine-ins,' which was profound. Slip-in 88's are merely over-bored 85.5's, resulting in a sealing surface so narrow you were liable to cut yourself. Slip-in 88's quickly became known as the most unreliable jugs ever made for the VW. They are wildly popular of course.

Which brings us to 92mm jugs. These happen to be thick-walled `machine-in' 88 barrels bored out to 92mm. And yes, they leak like a bitch. 94mm jugs, which are based on even thicker barrels (and can only be used on later-model crankcases because of it) actually have MORE sealing surface than 92's.

The quality of after-market VW parts has always been spotty at best. Right out of the box, upon blueprinting a set of barrels - - one step of which is to check their sealing surfaces for flatness - - many jugs were simply unacceptable. In most cases the UPPER sealing surface could be made acceptable by lapping the barrel on a surface plate upon #600 wet & dry paper flooded with kerosene. Flatting the lower sealing surface was more difficult and usually required machining. But the fact professional engine builders often lapped the cylinder's upper sealing surface gave rise to the Conventional Wisdom that EVERY KIND of lapping was a good idea. As you can see from the above, it's not. But all those instant experts who say it is have never paid much attention to reality.

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As for sealing rings, the proven alternative to gasket-less sealing surfaces is to use a gasket. (duh :-)

When Volkswagen introduced the Type IV engine with its 90mm jugs they finally bit the bullet and installed sealing rings. To keep the cost down the rings were STEEL, coated with pure aluminum. Alas, Volkswagen quickly learned that they could not be re-used, issuing a Service Bulletin to that effect. Properly annealed pure copper rings of the same thickness were an acceptable (but more expensive) substitute.

Copper sealing rings are now available for all commonly available cylinder diameters and are found in most professionally built four-stud VW engines. Their thickness effects the compression ratio and must be included in your calculations. Their use for this purpose isn't anything new, especially among air cooled engines, having been used on the Continental A40 (among others). Properly installed, especially with regard to annealing, copper sealing rings provide a reliable method of sealing a combustion chamber when the wall thickness of the barrel is less than optimum width.

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A converted VW is not a certified engine. Even if purchased ready to run, YOU are the Mechanic-in-Charge. For those who elect to assemble their own, without a good background in Volkswagen engines there's a tendency to do things without knowing why, other than `everybody' sez it's a good idea. Unfortunately, engineering is not subject to the democratic process; Robert's Rules of Order simply do not apply and the fact `everybody' does a particular thing is no guarantee it is the proper thing to do.

Even if you assemble the engine yourself most homebuilders will acquire only one engines-worth of experience in their lifetime. To ensure that lifetime is as long as it should be, you need to THINK FOR YOURSELF. It's important to know not only what others have done but WHY they have done so. If the best answer you can get is, `Because `everybody' does it that way,' I suggest you keep looking.

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This post is meant to be a general answer to a number of specific questions. Rather than answer each in detail I've offered some background that should allow you to answer those questions yourself.


PS - I just threw in that part about why the sky is blue :-)

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