platform115 wrote:...will it really cause a problem if I were to use 4?
Obviously some think it will and some don't. Don't expect to find consensus. Hopefully it'll be a moot point for you; if you find that you can't achieve proper endplay using three of the thickest shims and don't want to take any chances by thinking for yourself, you could have the mating surface of the flywheel machined off by a few thousandths.
I've wondered about this question as long as I've been working on VWs, and have found several plausible explanations for the reasoning behind the use of three shims. One that's appealing is that with three, one can stick to the bearing and one can stick to the flywheel and you'll still have one floating free between them....not that I've ever seen any stuck to either surface if the endplay was correct to begin with, but it sounds good.
Opinions on the importance of the exact number of shims differ, amongst experienced and respected builders. You won't have any trouble finding "authoritive" explanations from people who think they know the answer, but without a time machine and fluency in Technical German you probably aren't going to find any PROOF of the origin of the three-shim dogma.
MASSIVE TYPE IV wrote:no more than 3 shims, no less than 3 shims. If 3 shims of different thicknesses won't do it you have a case/bearing/flywheel problem. Find it.
Sharkey wrote:As long as you arrive at 0.003"-0.005" final clearance, it really doesn't matter how many shims you use -- as long as you use a minimum of three shims. The claim that you must use three shims is a myth.
This is the explanation I find the most appealing, although I've never found any "white paper" to support it:
amskeptic wrote:
Reduces shear loads on the oil. The shims serve as bearings to reduce the speed differential that the oil has to maintain a film against. There is no oil pressure feed between the shims unlike main and connecting rod bearings. At 6,000rpm with three shims you have no more than a 1,500rpm difference between two surfaces, well within oil's ability to maintain a film in a simple splash bath.
Note that if you follow this reasoning, four might be even better than three - but the vast majority of engines and other rotating assemblies which require endplay control use one shim...some use none. I think that the idea I postulated above, that it was done more for manufacturing economy than any other consideration, is worth considering.
