S6904-2RS Sealed ABEC-5 Ball Bearing With Si3N4 Ceramic Balls 20mm x 37mm x 9mm S6904-2RS Sealed ABEC-5 Ball Bearing With Si3N4 Ceramic Balls, S6904-2RS bearing is made of Stainless Steel with silicon nitride Ceramic Balls (Si3N4) and Nylon cage, S6904-2RS bearing is closed with rubber seals to protect the bearing from dust or any possible contamination, S6904-2RS bearing is sealed and self lubricated,the quality is set to EMQ (Premium ABEC-5 quality) to ensure high speeds, S6904-2RS bearing is lubricated with grease.