These are a direct OEM repalcement for YZ 250F/450F 2009-2025 Rear Axle. These rear axles are shot peened and heat treated to a Min. UTS 160Ksi (1103Mpa) for increased fatigue resistance. They are also PVD hardcoated to reduce galling. Due to the Mettec head design we are now providing the Alum block that are Hard Anodized as a complete package.