Most pistons run about .002"-.003" of skirt clearance. Measure the piston at about wrist pin height, and 90 degrees from the pin. Most pistons are "cam ground", meaning they're not round until they reach operating temperature. For instance, a standard piston for a 3 7/8" bore like a 283 V8 will measure about 3.8735" at the midpoint of the piston skirt. An unworn 283 standard bore will measure 3.875" from top to bottom. Most of the cylinder wear happens in the top 1 1/2" of ring travel, so even a worn out cylinder will measure correctly halfway down the stroke. The rings try to maintain contact with the cylinder surface by flexing twice each turn of the crank, so a ring will be changing diameter 83 times a second at 2500 RPM in a taper-worn bore. No wonder some re-ring jobs don't last!
Jerry
