Top End Piston Kit - 66.35 mm - Yamaha YZ250/X The piston is heat treated with a T-5 hardening and tempering cycle, The piston skirt is coated with a high-tech Molybdenum Disulphite coating for faster break-in time and increased piston life, Gravity-cast aluminum alloy (not forged) with 18% silicone content, Cast OE piston, Top end kit includes piston, rings, pins, circlips, top end gaskets, and wrist pin bearing