"What do you mean by piston to bore clearence. Is that the gap inbetween the two."
Aluminum expands about 1.7 times the rate of cast iron so there has to be room for that expansion. Rather then me typing this I'll just add this attachment! Remember: Different manufacturers and piston types(forged, cast,ETC,ETC, have different recommended clearances ) dont just open a repair manual and think that's correct!!!! you should always have the pistons before the block gets bored..That way the machine shop can do the final hone to get the exact piston clearance you need.. Most pistons you measure the skirts with a micrometer 90deg from the pin, just below the oil rings.
Measure the cylinder bore and subtract to get the clearance.
The piston is larger at the bottom of the skirt and smaller at the compression rings to allow for expansion of the piston dome. Pistons are also "cam-ground" (egg shaped, not perfectly round) as well so measuring somewhere other than the piston mfgr's. recommendations will give you an inaccurate number.
This is one way to check!