piston

if i got a stock b18a what size pistons can i put in it.

i think it's 81mm pistons. The pistons for all B series motors are the same size. (i think this is true, dont quote me on this, but im 80% sure that all B series motors have the same size pistons)

Yeah, he's right about the 81mm, all b-series motors, except the B20 (84mm) have the same bore.