What Has Caused This? Marks On Bore.

[r33_racer]

Isnt the top of the piston (around ring land/top ring) usually about 40 thou smaller then the skirt? Or does that part expand so much that it uses up the extra clearance whilst running at operating temps?

Maybe im wrong, I was always under the impression that piston size is based off skirt diameter...

[James_03]

Unless I worked that out incorrectly, thats only 2.8 thou of clearance. I assume the piston size is 87mm at the skirt? .07 is only .0028" Unless your measuring devices are not accurate enough???

What if the piston to bore was too tight, not too excessive? Would the rubbing show up much the same? or would it be in four corners instead of just two?

When I measured my wiseco's last night, I measured the top of the piston above the top compression ring. This came out at exactly 2.5 thou under 87mm, so with a 87mm bore it would have the correct piston to bore clearance.

Same with the old CP's. The top of the piston came out like 3 thou under 87mm. So they have put 2.8 thou over on the bore as you say, but more than likely 3 because I only had vernier calipers to measure with and there was a very slight carbon lip on the top of the bore. Plus the pistons were already 3 thou under 87 which obviously gives you in the area of 6 thou clearance.

Above the top compression ring was polished, which someone else mentioned as a tell tale sign.

I think that is what happend, I work in I.T. for a living so feel free anyone with more knowledge to chime in, but I think that's everyone's general line of thought.

[STATUS]

Measure it here;

[Blitz]

What if the piston to bore was too tight, not too excessive? Would the rubbing show up much the same? or would it be in four corners instead of just two?

Great question, I was just thinking this also.

[James_03]

Ok Ill check again tonight when I get home and re-measure it.

Edited March 2, 2010 by James_03

[DVS JEZ]

have you measured it yet???

[T04GTR]

did it rattle when cold?

if you pull all the rings off the pistons, slide one in. you shouldnt be able to flop it around in the bore.

or you could compare it with your new ones. by feel. not at all acurate but you will be able to see if one is tighter..