Sold in pairs. 21mm ID 40mm OD 20mm height. Micro cellular foam polyurethane elastomer shaft bump stop provides progressive suspension character even when combined with linear springs for a smooth approach to bottom out without disturbing the chassis stability. The level of the progressive character effecting the suspension compression as well as the wheel travel stopping point can be controlled by the height or number of bump stops in the stack on each shock shaft. If you are using a very soft coil spring because thats what gets you the traction you need over most surfaces, which causes coil bind or harshly impacting your rubber bump stop, you can regain car control by stacking up the progressive bump stops on the shock shaft until they touch the spring top hat at static ride height as seen on some OEM suspensions. The soft coil spring and large progressive elastomer stack combined can to emulate the progressive nature found in air suspesion where small high frequency bumps are smoothed out & the body is stable on a more gentle approach to end of travel, only without the added cost and complexity of air-ride.