THE JQRacing Rear Gearbox, (also known as bulkhead) includes a sway bar holder, and also a rear differential bearing insert. The gearbox is installed on the rear of the chassis, and is the component that houses the rear differential. I am super happy with the gearbox design. Specially the rear gearbox is a tough part to get right. It needs to be rigid enough to keep the gear mesh right under great torque from the drive train, impacts from hard landings, and the extreme heat from the engine. We were able to produce the gearbox so no shimming is required! Just pop the diff in, tighten the gearbox screws, and you are good to go! Due to the tight tolerances, some gearboxes leave the mesh slightly tight. This is not a problem. Just run the car and it will break in. However, make sure you push the diff pinion bearings in all the way!