Brand new high-speed ball-bearings produced particularly for XRAY. ID x OD x level (mm) = 5x12x4. Pair of 2 pieces. Top-quality specialist ball-bearings for a super-free, super-smooth drivetrain. These affordable ball-bearings are fitted with unique synthetic covers, and are also gently packed with unique oil for optimum smoothness, stamina, highest threshold and maximum performance. These ball-bearings do have more (and bigger) balls within the cage versus regular ball-bearings, making them more resilient under lots. Along side it covers are now an easy task to eliminate for fast and simple maintenance.