Due to the higher cylinder pressure of 4-stroke engines, pistons are generally made from forged pieces. To withstand the internal pressure of the engine, VERTEX pistons are forged from the highest quality aluminum alloy (2618). Each VERTEX piston features a MOS2 coating on the piston skirt to reduce wear. VERTEX is an OEM supplier to some of the largest motorcycle manufacturers, and many factory teams in MXGP rely on the performance of VERTEX. The piston kit includes a piston, piston ring(s), piston pin, and piston pin circlip! Piston configuration for 4-stroke: Pro Replica = forged piston matches the original form, Pro HC = forged special piston (higher compression and performance-enhancing).