I notice they sell flywheel shims. How do you know if you need them or not. I have a '67 396 w/ m21 trans. Do you use the shims if your going with an aftermarket Richmond or such?
Tom

I think their purpose is to compensate for material removed when resurfacing a flywheel.
I have however, used them to "adjust" for aftermarket flywheels which are almost always thinner than stockers.

What's the stock flywheel thickness? I'm putting an aftermarket on.
thanks, Tom

Bueller......Bueller....

Usually .975"-1.00" from the friction disc surface to the crank flange. The 2 aftermarket wheels I have measure .700" for a 15# aluminum one and .750" for a 30# steel one.

Is shimming critical or is there an acceptable margin for the difference?

Shimming by itself is not critical. What is critical is the relationship between the throwout bearing and the fingers of the clutch. Flywheel height is only one of the variables in the equation. The height of the throwout bearing, the height of the ball stud, the depth of the bellhousing, the height of the clutch fingers, the thickness of the clutch disk all affect the geometry of the linkage "system". You can juggle any and all of the variables to get to that "sweet" spot where it works as designed.

Flywheel shims in my experience typically come in .025" and .050" thicknesses. These dimensions are fairly small in the overall total "system". From what I've seen when you add @.100"-.125" in shims the flywheel ring gear comes painfully close to not clearing the starter pinion gear. So really not a lot to be gained there.

I've seen throwout bearings from different manufacturers vary .020"-.030" for the same application. (GM short, for example)

It always seems to me that every collection of parts I've put together results in a tolerance stack up that results in the linkage being out of whack. A thinner flywheel, a shorter throwout bearing, a scattershield and blockplate, and the newer diaphram pressure plates seem to have the fingers flattened out at the throwout bearing contact area all add up to longer distance from throwout bearing to clutch fingers. Any one of these by themselves would probably be OK, but when they all add up together It doesn't work well.

This is a really good article on linkage setup especially diagram #2. Just plug in some numbers for all the components in the diagram and see how it changes things.

http://www.classicchevy5speed.com/files/Clutch_Linkage_Installation_Tips.pdf