The Spidertrax Chromoly Full Floater End Cup Kit makes it easy to build your Spider 9 Housing into a competition grade full floating rear axle. Each cup is manufactured out of 4130 chromoly steel, and is designed to press fit onto a Spider 9 housing with 4″ (101.6mm) OD tube while accepting a Spidertrax Ultimate Unit Bearing (mounting hardware included). In addition, each cup has an integrated calliper mounting tab to easily install the bolt-on Spidertrax Full Floating Calliper Mount Kit. The Ultimate Unit Bearing and Calliper Mount Kit is sold separately. ???????These are not designed to be used with Pro Series Unit Bearings.