Welcome to Wallsky, I am happy to serve you Features: These miniature bearings are made of high quality bearing steel for low noise and long service life. They are double shielded to avoid contaminants entering and save lubricant. Bearings have a 3mm inner diameter, a 8mm outer diameter, and a 4mm thickness. These are great for small hobby 3mm shaft/rod projects such as 3D printer, quadcopter or ship model. There comes total 10 pieces miniature bearings which you may use for many different projects. Specifications: Material: Bearing steel Inner Diameter: 3mm / 0.12" Outer Diameter: 8mm / 0.31" Thickness: 4mm / 0.15" Quantity: 10pcs Package Includes: 10 x Bearings