15x21x4 Revolution Bearings
The Revolution Series bearings are a low friction bearing with several options to help satisfy anyone's desire. What makes this bearing unique is that it has a rubber seal on one side while having a metal shield on the other. Rubber seals are great protectors against dirt but the price is that they offer a little more friction due to the tighter seal. Now the performance of a metal shield is excellent but allows dirt to get in easily. So with the Revolution series, you run the rubber side towards the outside and the metal towards the inside. You get the protection of the rubber but the added performance by having one less rubber seal. But you don't have to stop there, if you are truly performance oriented and want the least amount of friction then you are in luck. You can quickly pop off the rubber seal with an X-acto and place the metal shield facing outward in your hubs. This is the best performance you could attain while maintaining some protection from dirt.
15x21x4 Halo Revolution bearing by Avid Racing Concepts is the standard outdrive bearing for the Team Associated RC8, Mugen MBX6, and the Losi LST2. Check your manual for the correct size for your vehicle if you are not certain.