Due to the higher cylinder pressure in 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 an MOS2 coating on the piston skirt to reduce wear. VERTEX is a first-choice supplier for some of the largest motorcycle manufacturers, with many factory teams in MXGP trusting in VERTEX's performance. The pistons come complete with piston ring(s), piston pin, and piston pin circlip! The 4-stroke version: Pro Replica = forged piston corresponds to the original form, Pro HC = forged special piston (higher compression and performance enhancing)