High Performance RC Engine bearing kits come with a high speed retainer that can handle 15% more Rpm's, In most cases the front bearings are also supplied with rubber seals (2RU) which helps to stop leakage through the front of the engine. Metal shields (ZZ) will be supplied in some cases where the sealed version is not available. These bearings have steel balls, a high speed retainer and a C3 fit. High Performance bearings can be used for any application seeking a bearing with a strong seal. 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.