Most Pistons (possibly all)come through with the clearance built into them for the bore size ordered. If you order a +.030 piston for a 350 Chevy than the bore size should be exactly 4.030 and the piston will be smaller by whatever the piston maker determines is the correct clearance. JE pistons come with a sheet that shows where the piston is measured just so the machinist can confirm the size. Take a set of verniers, dial calipers, or dial bore gage and measure the bore.
BSE Racing Engines