Due to the higher cylinder pressure of 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 is coated with MOS2 to reduce wear on the piston skirt. VERTEX is an original equipment manufacturer for some of the biggest motorcycle brands, and many factory teams in the MXGP trust the performance of VERTEX. The piston kit includes the piston, piston ring(s), piston pin, and piston pin lock! The 4-stroke Pro Replica piston is a forged piston that matches the original form. Pro HC is a forged special piston with higher compression and performance enhancement.