Grade 5 Chromium Steel Balls: High precision balls are within 5/1,000,000″ round. These are military level specification, used for precision equipment. R-64 hardness, of the highest grade. 52100 High Carbon Chromium Alloy Races: Vacuum de-gassing process for the purest chromium alloy steel, hardened to R-62, extremely tough and very durable. Polished to a mirror finish for ABEC-5 precision and noiseless movement. Nylon with graphite retainers are virtually frictionless while providing constant lubrication. To meet the ABEC-5 standard, bearing design must pass all tests, which include noise testing at high RPM, extremely close tolerances for I.D. and O.D., roundness of ball, trueness of the races and micro surface finishes of balls and races. C-3 Clearance This specification describes the internal clearance between the bearing and its race. Our standard is 11-21 micrometers where as others use 11-25 micrometers. Our bearings have higher standards of tolerance to ensure superior performance.