Keith Black Hypereutectic Piston Kit For Harley Twin Cam 88 Manufactured from high-strength, T-5 heat-treated, 390 hypereutectic aluminum alloy allows a tighter fit and longer piston and ring life. Features: Diamond-machined pistons are held to plus or minus one gram weight and less than one-half-thousandth-inch (.0005) tolerances from piston to piston Contains two pistons, pins, spiral-lock pin clips and Hastings moly rings KB pistons can be installed tighter than other pistons. This results in less piston rock and better ring seal. Due to the thermal conductivity and design of KB pistons, special attention is required to the top ring end gap.