Hi Ian, Modern pistons are usually cam ground or slightly oval. This is so that as the mass of metal expands around the gudgeon, when heated, the piston is circular in the bore. Similarly, when the rings lands are cut the head part is normally undercut slightly as the head of the piston is the hottest under combustion and expands a little more. Thus, when measuring piston to bore clearance it is measured between the gudgeons (not on the gudgeon) on the full skirt although the feeler guage is normally longer that the piston anyway. Make sure you check it at both ends of the bore. JP Pistons in Sth Aus recommend 0.004" for their Dougy pistons. This is a normal figure for pistons this size with no oil ring. Just up to this job myself. Regards John