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.