High-speed ball-bearings manufactured specifically for XRAY. ID x OD x thickness (mm) = 6x12x4. Group of 2 pieces. Top-quality professional ball-bearings for a super-free, super-smooth drivetrain. These finest quality ball-bearings are fitted with special plastic covers, and tend to be degreased and lightly oiled with AeroShell Fluid 12 oil for maximum smoothness, longevity, highest threshold and optimum performance. These ball-bearings do have more (and larger) balls inside the cage versus regular ball-bearings, making them a lot more resilient under lots. The side covers are easy to remove for fast and comfortable servicing. These ultra-high precision ball bearings are designed to withstand all extremes within track under race conditions as well as just fun racing. A "must have" product!