This purchase is for a set of frame and front axle sliders for the bike models above. The sliders feature the new design of a complete solid puck without a hollowed-out center. This increases the strength and contact surface to about 30%. The aluminum spacer bolts to the engine mount using the stainless steel bar. The slider puck is then screwed on to the spacer. Since this spacer is made of solid billet aluminum there is no need for re-tightening the engine bolt in the long run where you have to do so with plastic slider as the plastic would gradually give in and therefore making the bolt loose. The puck material are made of UHMW-PE (ultra high molecular weight polyethylene), a tough plastic with good strength, wear resistance and dimensional stability. It is more dimensional stable than Nylon and used in many industrial ball bearing and friction reducing applications. Compared to Delrin, it is softer but tougher. Delrin is harder but more brittle. It is an ideal material for slider since it can absorb more impact force onto itself and therefore reduce the impact on the frame and engine. The slider pucks can be inexpensively replaced in case of a low-side. We use highest grade commercially available 12.9(177,000 psi in tensile strength) wherever possible. Rest assured that you are getting the best material commercially available.