Ball Bearings 12x18x4 One Bearing One Ball Bearing for many applications, this is a popular size that could be used in many RC Cars and Trucks, or any application that uses this size 12mm x 18mm x 4mm. Bearings are made of Chrome Steel and can rotate up to 30000 rpm, each bearing has 2 metal shields to protect the bearing from dust or any possible contamination, also bearings are pre-lubricated with grease.