Pro Taper Race Spec MX Rear Sprockets Yamaha YZ65 (18-22). Race Spec (RS) Rear Sprockets are CNC machined to precise tolerances from high quality, heat treated 7075-T6 aluminum alloy. Painstakingly engineered, they achieve optimum load distribution, unsprung weight savings, and tooth endurance. The result is a durable, featherweight and feature-packed sprocket unequaled in the market.Heat treated, precision CNC machined- Pro Taper Race Spec MX Rear Sprockets Yamaha YZ65 (18-22).