This is the Size XS (aka Size K) bearing used in YoYoFactory yoyos such as the Mighty Flea and Loop 360/720 yoyos, Yomega Raider, etc. It may also be used by other yoyos.
The size of the bearing is as follows:
- Outer Diameter - 8mm
- Inner Diameter - 4mm
- Width - 3mm
Before buying this bearing make sure it will fit your yoyo, as it is not commonly used. Most yoyos these days use the standard Size C bearing.