Mar 25, 2008
69
0
Hello; I was wondering how much piston skirt to cylinder clearance is too much and if there is too much what will happen? Will the piston rock and catch the edge of the piston in a port causing "Real Catastophic" engine damage? Using a feeler gauge (I know a feeler gauge method is barbaric, but its all i have) I inserted the piston in the Cylinder Upside down while holding the piston with one finger against the intake side of the cylinder (almost in an effort to rock the piston as to generate the greatest possible clearance where I was measuring above the exhaust port) and started inserting blades as large as I could go. Well..... I made it all the way to .005" before the feeler gauge blade was really tight. should I rock the piston when measuring like this or just kinda hold it neutrally like by the piston pin, If I dont rock it in the bore then it is much harder to get even a .003 or .004" blade to slip in. By cocking the piston it seems like I would be better able to determine max clearance, Is this correct? Everyones help on this will be really appreciated.

Thanks, Stuart
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
Using a new or used piston? Never force a feeler gauge, a slight drag is it. The only real measurement you can get is the clearance when the piston skirt is in its normal position. Any groove felt means its worn out of spec, maybe even out of round. If you can force the piston over and get another measurement sure makes it iffy! You could use the rings the same way, but measure end gap, and look where there are any gaps. Iffy, it needs to be taken somewhere to be properly measured.
 

Ol'89r

LIFETIME SPONSOR
Jan 27, 2000
6,961
45
Gotta agree with foxforks. Take it somewhere and get it measured with a micrometer. A feeler gauge is not very accurate and your measurement will depend a lot on the width of the feeler gauge.

When you have too much piston clearance, the piston will rock and the skirt of the piston can crack and break off and that's when you get catastrophic damage.
 
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