Brand new high-speed ball-bearings produced specifically for XRAY. ID x OD x thickness (mm) = 5x12x4. Group of 2 pieces. Top-quality expert ball-bearings for a super-free, super-smooth drivetrain. These finest quality ball-bearings are fitted with special plastic covers, and so are gently full of special oil for optimum smoothness, long life, highest threshold and optimum performance. These ball-bearings have more (and larger) balls inside the cage compared to the regular ball-bearings, making them even more resilient under lots. Along side it covers are easy to eliminate for quick and easy upkeep.