This purchase is for a pair of rear axle sliders for the models above. The sliders are made of UHMW-PE (ultra high molecular weight polyethylene), a tough plastic with good strength, wear resistance and dimensional stability. It is more dimensionally stable than Nylon and used in many industrial ball bearing and friction reducing applications. The aluminum spacers are made of 6061-T6 aluminum alloy. Rest assured that you are getting the best material commercially available.