S61904 Open Ball Bearing ABEC-7 Quality with Si3N4 Ceramic Balls (Silicon Nitride) 20mm x 37mm x 9mm S61904 Open Stainless Steel, radial, deep groove ball bearing,S61904 bearing is made of Stainless Steel with Ceramic Balls (Si3N4) and Nylon cage,the quality is set to EMQ (ABEC-7 quality) to ensure high speed,