Due to the increased cylinder pressure of 4-stroke engines, pistons are generally made from forged materials. To withstand the internal pressure of the motor, VERTEX pistons are forged from the highest quality aluminum alloy (2618). To reduce wear, each VERTEX piston features an MOS2 coating on the piston skirt. VERTEX is a leading original equipment manufacturer for some of the largest motorcycle manufacturers, and many factory teams in the MXGP trust the performance of VERTEX. The piston kit includes piston ring(s), piston pin, and piston pin clips. The Pro Replica 4-stroke piston design = forged piston corresponding to the original form, Pro HC = forged special piston (higher compression and performance-enhancing).