if you NEED a rebore then get as little as possible, the walls on the 4e are very, very thin. the max you can go is 75mm, and i think thats too much. 74.5mm if you can get away with it. and as si said, standard is 74mm. if you get it bored to 74.5, you get 74.5mm pistons. when ordering pistons they take into consideration the ring clearances.