This is the Traxxas TRX .12 and .15 Engine Stock Replacement One-Way Bearing.  This bearing is located on the starter shaft.

Features
Steel construction
Roller bearing design

Includes
One One-Way Bearing

Requires
Nothing.  This is a direct replacement part

Specs
Inner bearing diameter: 6mm (0.24")

Comments
This bearing is unique to this engine, and cannot be substituted by another bearing