These forged pistons are designed for use in S&S? Big Bore Hot Set Up Kits? for HD? Evolution? big twin engines, and S&S? 3-5/8″ bore engines with S&S? Super Stock? cylinder heads and 4-1/4�, 4-1/2″ and 4-5/8″ stroke engines with stock diameter (8-1/2″) flywheels. 3-5/8″ bore, standard size.