I've been reading Wayne Dempsey's "rebuilding your 911 engine 1965 - 1989" book. Really good stuff! However, I do have one question in relation to piston & valve clearance; If I understand him correctly you add the copper shims at the bottom of the barrel to adjust deck height clearance. Does adding copper shims also lower the compression ratio as well? If so, is there any ratio formula for shim thikness to ratio decline? Then if you achieve proper deck height would piston / valve clearance only change if a head was machined?
Yes, the CR drops as shims are added. However, you also must consider the reduction in chamber volume that results from surfacing the head. Lotsa math here .................. To reduce the problems caused by stacking shims, thick ones are available. Generally, a good machinist will record the cut and you simply add the appropriate shims. Remember, all the head cuts MUST be the same so all the barrels end up having the same deck height. The Cap'n
Krusty is spot on. Though I'll add a little bit more. You don't shim the cyl height to correct for valve to piston clearance. Thats done w/ proper cams and/or valve relief cutouts in the pistons. The deck height takes care of the piston to HEAD clearance. Almost always out at the very edge. W/o enough deck height the pistons will hit the heads.
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