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). To reduce wear, each VERTEX piston features an 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 also rely on VERTEX's performance. The kit includes a piston along with piston ring(s), piston pin, and piston pin clip! The Pro Replica piston is forged to match the original form, while the Pro HC piston is a forged special piston that offers higher compression and performance enhancement.