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Registered User
Join Date: Aug 2004
Location: SF Bay, CA
Posts: 39
Exclamation Help! Piston to valve clearance ???

OK, today's question is "Why don't I have enough piston to valve clearance?".

My 73S ran fine, but leaked spectacularly, so I embarked on "fix your oil leaks" and a few other things. To the best of my knowledge, it's stock and untouched at 120K miles.

Teardown - OK, everything looked good, except for the loose fittings and seriously fried seals.

Cams back in, timing went OK, with the left cam at ~5.28mm and the right at ~5.23 at overlap, with 0.1mm clearance. I know there's a debate here about what's close enough, but bear with me - they're both between the 5.0 to 5.4mm range.

Next - install the #1 and #4 exhaust rockers and check the piston to valve clearance. I did this by (1) going to "both valves closed" TDC, (2) screwing the valve adjustment screws in until they touched and then another 2mm (2 turns) for exhaust and 1.5mm for intake, (3) slowly and gently turning the crank until - **** - it won't turn any more.

Much rechecking and note-taking then ensued, and I deduced that the left intake had 0.5-1.0mm clearance, the left exhaust 1.0-1.5mm, right intake 1.5-2.0mm, and right exhaust 1.0-1.5mm. I know that these are rough numbers, and I'll check them again and try to refine them, but clearly, I have clearance issues.

So, what's the scoop? No machine work, no disassembly beyond R&R of rockers and cams. It was (apparently) fine before, the timing is at least within range, but there are clearance issues now. What am I doing wrong? Do I need to adjust the timing to one end of the range or the other to get more clearance?

My cams (presumed stock) have the dots and an "S" stamped on the right one, and "01" stamped on the left one.

Old 08-29-2004, 10:55 AM
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Registered User
Join Date: May 2003
Posts: 3,231

Sorry to hear about your clearance difficulties. It seems that with "S" cams these clearances are very tight. I had to have my piston pockets cut deeper on my 2.7 for the same reason. For starters the clearances you see in Waynes book and on this web site are more conservative than Porsche used. This is for good reason as many of the engines we are dealing with are for racing or are made with aftermarket parts. Porsche only references the intake valve clearance on one page of it's workshop manual. It says the clearance should not be less than .8mm. There is no reference for the exhaust clearance. In Bruce Andersons book he recommends 1.5mm minimum. The exhaust is more critical because the piston is "chasing" the exhaust valve so any float at all or expansion of parts due to heat will reduce this clearance. Personally I would accept a minimum or 1mm on intake and 1.5mm on exhaust. If you can adjust the cam timing to achieve that you may be ok.

Other options are to have the pistons cut (many machine shops can do this), or putting a thicker base gasket on the cylinders.

Good luck on your project.

72 Carrera RS replica, Spec 911 racer
Old 08-29-2004, 11:28 AM
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Registered User
Join Date: Aug 2004
Location: SF Bay, CA
Posts: 39
Question Another thought

Thanks. I thought of something else to check.

I timed my cams with the idler arms clamped in to keep the chain quite tight - how tight, I don't really know. After timing, I installed the tensioners (pressure fed) and the exhaust valves, and then did the clearance check. I wonder what my timing is like with the tensioners? Might it be significantly different than with the idlers clamped? The chain seems - to first order - about as tight with the tensioners, but I don't really know.

It poses an interesting question. Should the timing be as accurate as possible (or maybe with the intake lift toward the lower end of the range, which seems to provide more clearance when checked later) with the idler clamped, or with the tensioners installed? The latter is obviously how the car runs, but when the engine is running, I supose that the chain is being pulled and quite taut. Hmmmm.

I'll check the timing again, with the tensioners, tomorrow morning. It's too hot in the garage now, and we've got to take the kids to the movies.

Any advice, encouragement, and good jokes will be appreciated.

Old 08-29-2004, 01:26 PM
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