S61901 Sealed Ball Bearing ABEC-5 Quality with Si3N4 Ceramic Balls (Silicon Nitride) 12mm x 24mm x 6mm S61901 Open, radial and deep groove ball bearing,bearing is made of Stainless Steel with Ceramic Balls (Si3N4) and Stainless steel cage,the quality is set to EMQ (ABEC-5 quality) to ensure high speed,inner diameter is 12mm, outer diameter is 24mm and width is 6mm.