ZERØ ceramic bearings feature ceramic balls and steel rings. The silicon nitride balls (Si3N4) are grade 3 and are the roundest balls you can purchase without a military license. The rings are treated with a black oxide layer that helps prevent rust in the bearing. After this treatment, the race rings are polished to a high gloss. Finally, they undergo cryogenic treatment before assembly to enhance their strength. We also use VV silicone seals, the lightest seals on the market. Dimensions: 10x26x8.