Finally go inverted without weeble wobble!!! This upgrade replaces the stock o-rings...pack of four (set of two complete sets). The advantages of these dampeners is simple. They are able to produce a more responsive and tighter head dampening response by filling the entire dampening area of the head block with material. The dampeners are designed specifically for the T-Rex 450XL/SE V1 /SA head block and spindle, giving a tight dampening solution without needing to over-compress the rubber with spacers. Also, because of their shape and material, the Pro T-Rex Head Dampeners are able to last much longer.