Between the flywheel thrust surface and the #1 main bearing thrust surface in the engine block there are usually three shims that determine how much crankshaft axial movement, or “end play,” will result. It is not necessary to have three shims. VW could have designed the system to only take one, like some Porsche engines use. For whatever reason, probably production related, VW chose three as the magic number. However, in some cases it might be necessary to use only two, or maybe four when you are replacing the flywheel—and that is totally okay!
When VW built these engines, they had total control over all the parts, which were (of course) all brand-new. When working on a used engine with a reconditioned OE German flywheel or a new flywheel, the dimensions of the main bearing, engine block, and flywheel can all stack up in such a way that makes it impossible to get the right end play with three shims. If this happens: Don’t panic! It is perfectly okay to use one more shim in the case of a used flywheel whose thrust surface has been reground, or one less in the case of a new flywheel where the thrust surface is on the high side. Bottom line: With all the shim thicknesses available, you should not have any trouble getting the right end play—you just might have to stray from the original three-shim system.
However, you might say, “Lucas, in order to get the end play even close to correct, I can’t even rotate the crankshaft! The flywheel must be bad!” If you run into a situation on an early 1983-85, 1.9 liter-type waterboxer (DH engine block code, for example) where you add shims to the point you get binding and can’t even rotate the crankshaft, but there is still end play present, you have a bigger problem. What's happening is the #1 main bearing, which is a one-piece bearing in the early waterboxer, has come loose in the engine block, and it is moving in/out of the engine block itself. If it is moving A LOT, it is just time for an overhaul, plain and simple. But if it is only moving a little, you could run it for a while longer by just adding the movement to your factor specification and calling it macaroni. The rule of thumb is if the bearing is moving less than the end play setting specification, you might be able to get away with it—in fact, it could run that way for many years. But don’t call me if it doesn't!
But, you say, “I don’t have a 1.9 engine. I have a 2.1 liter-type (1986-91 MV or DJ) engine, and one of the shims is a very special. It has a larger OD and only comes in one thickness! What do you say about that, Lucas?!” There is a lot of misunderstanding surrounding the mysterious, oversized shim VW used on the 2.1 liter engine that is different than the other two, which are the same as before. This shim appeared at the same time VW changed the #1 main bearing from a one-piece bearing to a 3-piece bearing, with separate thrust washers. VW redesigned this bearing with separate thrust washers to eliminate the problem the one-piece bearing had: coming loose in the engine block. The purpose of this special shim was to work in conjunction with another piece that fits between the flywheel seal and engine block.
The ID of this part pushes up against the OD of the special shim, and they work together to retain the main bearing thrust washer in place when the flywheel is removed. These two items are B and D in this diagram:
The special large OD shim (D) was available in only one thickness, so people often panic about not being able to set the correct end play due to variations in their new or reground flywheel or flex plate necessitating thicknesses for the two other shims that don’t exist. But here is the thing: These parts are not critical for normal operation. They were simply placed there for ease of production at the factory—to keep the thrust washer (A) in place between the time the engine went from the previous work station to the one where the flywheel or flex plate was installed. It is 100% okay to delete both B and D when installing a new flywheel seal, and use two, three, or four of the regular shims (C) which are available in a multitude of thicknesses to get the end play right on the money. However: Don’t make the rookie mistake! Make SURE the thrust washer stays in place—which is CRITICAL!
If you end up in a situation with a 3-piece bearing and end up without enough end play—no matter how thin or few shims you use—WATCH OUT! Whether you put back the large OD shim and retaining ring or not (a little dab of grease works great, too), you have to MAKE SURE the thrust washer is IN PLACE. What commonly happens is it falls out of place, the flywheel is bolted back on and crushes the out-of-place thrust washer. If you STOP here and replace it, it is not a huge deal. But if you simply use thinner shims to “make it work,” put it all back together, and run the engine: you can be in for BIG trouble, including ruining the engine block!
If you get in a situation where your crankshaft end play cannot be set with three shims simply because the new or reground flywheel you are using in place of the one that came off requires something other than three shims: Don’t panic! You (and your waterboxer) can live with fewer or more! It’s all good!