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 has 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 the MXGP trust VERTEX for their performance needs. The piston kit includes piston rings, piston pin, and piston pin lock! 4-stroke piston design: Pro Replica = forged piston corresponds to the original shape, Pro HC = forged special piston (higher compression and performance-enhancing).