Due to the higher cylinder pressure in 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 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 MXGP rely on VERTEX for their performance. The package includes piston(s), piston ring(s), piston pin, and piston pin security! 4-stroke piston version: Pro Replica = forged piston corresponds to original form, Pro HC = forged special piston (higher compression and performance-enhancing).