Cylinder / head shims

dbrody · 01-09-2002, 03:52 PM

I'm rebuilding a 912 engine and one of the cylinders has put a nice groove in the head. I'm thinking about using a shim -- any suggestions?

Dave

Green 912 · 01-09-2002, 06:23 PM

Don’t.... the head needs to be resurfaced. (Fly cut) The head mating surfaces need to be flat as well as parallel so it will sit flat on the jug as well as the same height jug to jug otherwise you might/will get leaks. After you cut the head you can add shims under the jug between the jug and the case to make up for the “lost” height from the machine work.

dbrody · 01-09-2002, 09:14 PM

Any idea on where to get the shims?

Green 912 · 01-10-2002, 09:11 AM

Use the copper sealing gaskets that are used between the jug and the case. You can get the rings from the VW performance suppliers; they are a bit thicker and a lot less expensive than Porsche part.

When I reassemble a motor I calculate the # of rings needed to get the clearance and compression that I want then always install 1 more than needed. After I tighten everything up and test all the values I can then just loosen the head and jug and clip the “extra” seal ring and pull it out without having to pull the jugs off. If the “extra” ring is needed then I am home free to start and don’t have to pull the jugs off to add a ring. Use a combo of after market and OEM to really dial in the compression you want. 1 OEM and 1 after market + the extra shim and you can cut the one(s) that gives the best overall values you want. 2 thick 1 thin-- 2 thick-- 1 thick 1 thin-- 1 thick-- 1 thin. This goes from lower to higher compression. It is best to use the least # needed but you have to have at least one.

Remember to always have the same # and thickness of shims on both jugs per head and set the compression based on the “smallest” combustion chamber.