Piston kits include set of pistons, rings, wristpins, and clips These forged, 3-5/8" big bore flat topped pistons are used in S&S Cycle 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.