i don't know for sure, but i would expect it to follow a good rule of thumb of about
.001" per inch of diameter.
so for a 3" piston, .003"
maybe a bit more if the engine is going to be run under heavy loading, or
if it is running cool and then asked to take a max loading before the engine is up to normalized temps
in which case maybe an extra thou or two, but this would be for an unusual application in my opinion.
why do you ask? are you looking for a max limit? such as after a rehone of the cylinder?
also the piston (if made properly) will not be round but more like an oval in shape, wider perpendicular to the
wristpin (grudeon pin for our english purest's), so be sure to measure for clearance in the center of the skirt
90 degrees from the pin.
(just in case you didn't already know, if you already knew that,, then nevermind)
bob g