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). Each VERTEX piston features a MOS2 coating on the piston skirt to reduce wear. VERTEX is a first-time equipment supplier for some of the largest motorcycle manufacturers, and many factory teams in the MXGP rely on VERTEX performance. The piston kit includes piston ring(s), piston pin, and piston pin safety clip! Piston Type for 4-Stroke: Pro Replica = forged piston corresponds to the original shape, Pro HC = forged special piston (higher compression and performance-boosting).