HIGH QUALITY: Bearings are made of high quality steel for durability and long life. DOUBLE SHIELDED: Mini ball bearings are double shielded to prevent contaminant ingress and save lubricant. U-GROOVE: The bearings have a 1.8mm deep U-groove that typically rolls on a rail or track. PERFECT FOR LINEAR SYSTEM: They are specialty bearings that are used in many linear motion systems. Model: U624ZZ Inner Diameter: 4mm / 0.15inch Outer Diameter: 13mm / 0.51inch Height: 7mm / 0.27inch U-Groove Width: 5.2mm / 0.2inch U-Groove Depth: 1.8mm / 0.07inch inch Color: Silver Material: Steel Size: 13 x 7 x 4 mm Package Contents: 100 x Bearings Only the above package content, other products are not included. Note: Light reflection and different displays may cause the color of the item in a little different from the real thing. The measurement allowed error is +/- 1-3 cm.