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chancecasey's Avatar
 
Join Date: Sep 2005
Location: cupertino, ca
Posts: 320
3.6L piston to cylinder clearance limits

Just mic'd my p/c's - wondering if they are good to go for another 100K miles...

Cylinders range from 100.02mm to 100.05mm.
The group, new, were between 100.021mm and 100.028mm.
The largest variance in dimension on a cylinder (difference in diamater across 4 measurements) was .02mm (.0008 inches) so I'm good on roundness - I've heard (but not read) that anything under .025 inches in variance is good here.

Pistons range from 99.94mm to 99.96mm.
The group, new, were between 99.991mm to 100.001mm.

The manual says p/c clearance, new, should be .02-.03mm

What I cannot find is, what an acceptable p/c clearance is to re-use the set. From my calculations, I'm now at .05mm gap on average between piston and cylinder - which is .002 inches. Is that good enough to get me another 100K miles down the road, assuming "everything else is normal"?
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'90 C2 Cab - Temporarily out of service

Last edited by chancecasey; 02-04-2006 at 03:05 PM..
Old 02-04-2006, 02:09 PM
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Join Date: Apr 2001
Location: Vancouver, BC, Canada
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I don't know if I can help you but I do have a few questions:

1 - How and where on the piston did you measure? I found it really difficult to get the accuracies into the tenths of thousands of an inch since your hand heat alone can throw the micrometer off and the position on the piston is critical since they tend to be tapered.

2 - The group new measurements are from the spec book for the size of piston you have (.e. +1. +2)?

3 - The spec books tell us that the acceptable gap is 0.02 - 0.03. I think this is not only the new range but the range at any time...i.e. the gap should not be larger than 0.03.

The biggest problem I found is getting the measurements spot on...its really really easy to be off just a bit and then all of a sudden you gap is too large. Even the guys in the machine shop varied. Have you calibrated the mics that were used?

What is the mileage of the P/C set?

Cheers,

Mike
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97 993TT Arena Red - "Scarlett"
Old 02-07-2006, 04:15 PM
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Yeah, in retrospect I suppose my question about future mileage is kind of assenine. I measured the piston appx 10cm from the bottom of the skirt, on the side 90 degrees from the wrist pin boss. I took the measurement at the same height on the skirt as the bottom of the wrist pin boss hole. That is where the manual says to measure I think.

My group is 3. Don't remember a +/-.

I saw .02 - .03mm as well, but this is definitely new - because that's .0008 to .0016 inches. Anything less than a thousandth of an inch is simply incredible. Brand new JEs with various aftermarket cyls have gaps up to .005 inches I believe. I know, different equipment means different gaps but still. I'm taking a cue from Henry at Supertec about the cylinder roundness - I figure if they can be 2 thousandths "egged", then my calculated clearance ( +/- 100% of the measurement itself) is still probably pretty darn good.

The current mileage is 100K.

I hear you on taking the mic readings. I zeroed and re-mic'd everything 2 or 3 times, and my margin of error seemed to be +/- .02mm or .0008 inches - which is the calculated gap itself, so this is pretty ridiculous stuff. I have done some time in a physics lab a long time ago so I know how to use one - but expecting accuracy better than 1 thousandth with your hands and a caliper-type device is unrealistic no matter how precise the instrument is - unless you have some kind of jig or other helper device to hold things in place. But then it's not exactly "by hand"

Anyway, FWIW I'm goign to run them and see how it goes. I got my rods on my crank last night. YAY - finally going in the ASSEMBLY direction on this beast! I'll keep everyone posted on how it works out over the miles. Wish me luck!
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'90 C2 Cab - Temporarily out of service
Old 02-07-2006, 06:02 PM
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