S6904 Open Ball Bearing ABEC-7 Quality with Si3N4 Ceramic Balls (Silicon Nitride) 20 x 37 x 9 mm S6904 Open Stainless Steel, radial and deep groove ball bearing,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,inner diameter is 20mm, outer diameter is 37mm and width is 9mm