Due to the higher cylinder pressure of 4-stroke engines, the piston is 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, every VERTEX piston features a MOS2 coating on the piston skirt. VERTEX is an OEM supplier for some of the largest motorcycle manufacturers, and many factory teams in the MXGP trust in the performance of VERTEX. The kit includes the piston, piston ring(s), wrist pin, and circlips. Piston version for 4-stroke: Pro Replica = forged piston corresponding to the original form, Pro HC = forged special piston (higher compression and performance enhancing).