30 x Deep groove ball bearings: 20* 624ZZ + 10* 604zz Single row deep groove ball bearings. Have 2 metal to protect the bearing from dust or any possible contamination. These bearings are pre-lubricated with grease. Deep groove geometry for high speeds and supporting both radial and loads. Chrome steel for durability and resistance to deformity under heavy loads Metal on both sides of the bearing to keep lubricant in and contaminants out. Lubricated at manufacturer for no additional lubrication required. This 604ZZ 4x12x4-millimeter shielded miniature ball bearing has deep groove geometry for high speeds and supporting both radial and loads. Its chrome steel construction makes it and to deformity under heavy loads. Material : Carbon Steel/Bearing Steel Size: inner diameter: 4mm outer diameter: 13mm thickness: 5mm, Weight: 0.19069985663 Pounds, Manufacturer: Oewwthy