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.