This is an optional Avid RC 2.5x6x3mm "Tungsten Carbide" Associated/TLR Differential Thrust Bearing. This thrust bearing makes working on your diff much easier. You no longer have to worry about losing a diff ball or getting covered in grease when you rebuild your diff! Tungsten Carbide thrust bearings are more true than steel and slightly harder, but are not as smooth as ceramic, allowing the balls to grab the ring surface easily making it easy to set the diff. These are recommended for most racers.