These forged, 3-5/8" big bore flat topped pistons are used in S&S Sidewinder kits and Hot Set Up Kits for HD Evolution big twins with stock cylinder heads and 4-1/4" and 4-1/2" stroke engines with stock diameter (8-1/2") flywheels and 4-3/4" stroke engines with 8-1/4" diameter flywheels. 3-5/8" bore.