6 x 15 x 5 Stainless Steel Ball Bearing with Si3N4 Ceramic Balls (Silicon Nitride) 6mm x 15mm x 5mm 6 x 15 x 5 Stainless Steel Ball Bearing, Quality is ABEC-5, Bearing is made of Stainless Steel, each bearing has 2 metal shields to protect the bearing from dust or any possible contamination.