5 x 10 x 4 Flanged Sealed Ball Bearings, this is a popular size that could be used in many RC Cars and Trucks or any application that uses this size 5mm x 10mm x 4mm with Flange. Bearings are made of Chrome Steel, each bearing has 2 Seals to protect the bearing from dust or any possible contamination, also bearings are pre-lubricated with grease.