Given the higher cylinder pressure in 4-stroke engines, the VERTEX piston is generally made from forged pieces. To withstand the internal pressure of the engine, the 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 (OEM) for some of the largest motorcycle manufacturers, and many factory teams in the MXGP trust in the performance of VERTEX. The VERTEX Top-End Piston Kit saves time and money, as this set includes everything needed for a piston replacement. Delivery includes: piston kit with cylinder gasket set and timing chain! Piston design for 4-stroke: Pro Replica = forged piston corresponds to original shape, Pro HC = forged special piston (higher compression and performance enhancing).