This is a Protek R/C Ceramic 14x25.4x6mm "MX-Speed" Rear Engine Bearing, and is intended for use with any nitro engine that requires a 14x25.4x6mm rear bearing. ProTek MX-Speed engine bearings offer decreased resistance, increased speed, load handling capabilities and extended bearing life. The bearing dimensions are 14mm inner, 25.4mm outer and 6mm thick. This bearing is open to allow oil to lubricate the bearings during use. This bearing will also fit any other .21 to .28 competition engine that uses the same bearing dimensions.

They also have 2 distinct advantages when compared to other bearings.
Precision Ball Races: A unique bearing race design is used that increases load capabilities, centering the balls in the middle of bearing race. This allows the bearing to continue to rotate smoothly when the crankshaft is subjected to harsh side and radial loads.
PA46 Nylon Cage: A stronger, thicker bearing cage material has been developed that allows the bearings to withstand high nitro content fuel and excessive temperatures. The reinforced PA46 nylon has been subjected to Heat Distortion Temperatures of 554° without failure.
NOTE: This bearing should be installed so that the balls face the rear of the engine (towards the backplate), in order to allow proper lubrication.