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MBradbury MBradbury is offline
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Join Date: Jan 2001
Location: Troy, Michigan, USA
Posts: 62
Exclamation Today's self doubt!

Hi,

Background... building a 3.2L short stroke from a '83 3.0L SC. Other mods include 964 profile cams reground from the original SC cams. The pistons and barrels were sourced from Andial who also machined the heads to accomodate the larger diameter 98mm pistons. Andial also supplied sheet steel head to cylinder seals (porsche part 930 104 381 01). All of these parts and work are laid out in the photo below.



The problem... almost completed assembly to the point of bolting down the cam tower on the RHS, when it dawned on me that the head seals had a smaller inside diameter than the pistons. This has got me doubting I have the correct seals. I have drawn what I think is going on below. Surely, if this is the case, when I was busy measuring piston to head clearance, I was actually measuring piston to seal clearance. I know the seals are only 0.02", so I still gained 0.10" from the head machining, but why aren't the seal inside diameter bigger than the piston diameter? Is it because of the bastard piston diameter of 98mm? To get to the 0.050" (1.27mm) piston to head clearance I had to add 0.050" shims under the cylinders. If the seal had a larger inside diameter, I would have only needed 0.030" shims. The excessive shiming under the cylinders is due to case machining and to compensate for a difference of 0.015" in cylinder height less than standard SC barrels. The upside of this setup is I get approximately an additional 0.020" valve to piston clearance. Although, clearance on the valves didn't seem to be a problem.



Has anybody out there built a 3.2L short stroke, and did they use similar head seals?

Regards
Mark Bradbury
1983 911 SC
1988 Carrera (long term '73 RS lookalike project)
Old 12-01-2003, 06:34 PM
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