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JeremyD's Avatar
 
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What's considered acceptable tolerances?

My cylinders that I bought used (won't do that again) - but just back from the plater have been measured at 98mm but + .001" to .002" on one of the jugs.

I've been told by a few people that I should get these replated, but was recently told by someone I have respect for that with new quality rings, I should be fine.

is there a consensus?
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Old 02-01-2006, 06:57 AM
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This can be an interim post until the experts chime in.

Depending on the piston, 0.002" oversize could already be too much considering the piston-to-cylinder clearance would normally be around 0.001 - 0.0015" (Mahle). Aftermarket pistons usually run larger clearances so you might be okay. Maybe.

I would measure the piston OD, subtract from cylinder ID, then determine if the oil clearance is within "spec". If the cylinder has to be/can be redone, the shop should let you know.

Sherwood
Old 02-01-2006, 05:50 PM
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From the Porsche spec book for 84-87 911: max difference between cylinder diameter & piston diameter is .12mm.

Not sure the relationship because 98mm is bigger than stock ones.
As Sherwood said, I also hear that some after market pistons/cylinders requires bigger tolerance.
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Old 02-01-2006, 06:39 PM
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You have to know what the piston spec is befor you know if this is an issue. The factory stuff is speced on groups, both piston and cylinder. Measure the OD of the piston first then drop us those numbers.
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Old 02-01-2006, 06:42 PM
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We are finding that many replated cylinders are out of round so if yours are just a little large, consider yourself lucky.

Measure your pistons and see if you can fit a piston within specs. The pistons are not exactly the same size and you may get lucky.
Then again if you feel lucky just run them.

You could have the smallest of your cylinders honed to match your largest. Then have a set of pistons made to match the new size. You are talking about a very small amount but if you want, you can make them better.
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Old 02-01-2006, 07:05 PM
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