R4ZZ Shielded Ball Bearings, Bearing is Metal, this is a popular size that could be used in many Radio Control Cars and trucks and applications that uses this size 1/4" x 5/8" x 0.196" inch. Bearings are made of Chrome Steel, each bearing has 2 metal shields to protect the bearing from dust or any possible contamination, also bearings are pre-lubricated with grease.