Needing to use a lot of old pistons of unknown material (aluminum compound) I
had a tried and true method of determining the bore clearance.
I would boil a coffee can of water and slip a piston into the water, wait a
couple minutes for the piston to heat up , then with gloves (desirable) and a
suitable retractor remove the piston form the water and mic the piston across
the skirt bottom both ways. Mic again just under ring grooves both ways. By
comparing the results of the piston at room temperature and those from the
boiling water you have the expansion rate. Add .003" to the heated piston
dimensions and you will in most cases be right on the money for racing
clearance.
"Never be beaten by Equipment"
Kas Kastner
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