This is an optional Avid RC 2.6x6x3mm "Steel" Kyosho/Yokomo 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! The steel thrust bearing delivers an economical, easy to adjust diff. However, they will require more frequent replacement and are not as smooth as carbide or ceramic.