Ceramic High Performance bearing kits come with a high speed retainer that can handle 15% more Rpm's. In most cases the front bearings are supplied with rubber seals (2RU) which helps stop leakage through the front of the engine. Metal shields (ZZ) will be supplied in some cases where the sealed version is not availalble. These bearings have ceramic balls, a high speed retainer and a C3 fit. Ceramic hybrids are lighter, capable of achieving higher RPM’s and sustaining higher operating temperatures than traditional bearings. Whenever using a ceramic hybrid bearing in an RC Engine, we suggest leaving both seals on for the front bearing and one seal facing the inside of the motor on the rear bearing. This protects the RC engine from possible bearing failure. When selecting your Engine Bearing kit please check to be sure you are getting the proper sized bearings for your engine or contact your Boca Bearing Sales rep for details.