S6801 Open Ball Bearing Premium ABEC-5 Quality with Si3N4 Ceramic Balls (Silicon Nitride) 12 x 21 x 5 mm S6801 Open 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 (Premium ABEC-5 quality) to ensure high speed,inner diameter is 12mm, outer diameter is 21mm and width is 5mm.