Due to the higher cylinder pressure of 4-stroke engines, the 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). To reduce wear, each VERTEX piston features a MOS2 coating on the piston skirt. VERTEX is an 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 rings, piston pin, and piston pin lock! Piston version 4-stroke: Pro Replica = forged piston corresponding to the original form, Pro HC = forged special piston (higher compression and performance-enhancing).