SR4ZZ Stainless Steel Dry Premium ABEC-5 Ball Bearing with 6 Balls Si3N4 Ceramic Balls (Silicon Nitride) 1/4" x 5/8" x 0.196" SR4ZZ Stainless Steel Ball Bearing with 6 Balls Ceramic Balls Premium ABEC-5 quality, Bearing is made of Stainless Steel, each bearing has 2 metal shields to protect the bearing from dust or any possible contamination.