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.