The Spidertrax 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 designed to press fit onto the Spider 9 housing with 3½” (88.9mm) OD tube and accept the Spidertrax Ultimate Unit Bearing (mounting hardware included). In addition, each cup has an integrated caliper mounting tab to easily install the bolt-on Spidertrax Full Floating Caliper Mount Kit. The Ultimate Unit Bearing and Caliper Mount Kit is sold separately. ???????These are not designed to be used with Pro Series Unit Bearings.