17287-2RS Sealed Ball Bearing with Si3N4 Ceramic Balls Silicon Nitride 17mm x 28mm x 7mm Made for bicycles 17287-2RS Sealed Ball Bearing with Si3N4 Ceramic Balls, bearing is made of Chrome Steel with Ceramic Balls (Si3N4),17287-2RS bearing is closed with rubber seals to protect the bearing from dust or any possible contamination,bearing is sealed and self lubricated,17287-2RS bearing is lubricated with grease.