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Brake Rotor Surface & Wear Inspection

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WARNING: Avoid grinding brake linings, sanding brake linings or cleaning wheel brake parts with a dry brush or with compressed air, when servicing wheel brake parts. Some models or aftermarket brake parts may contain asbestos fibers which can become airborne in dust. Breathing dust with asbestos fibers may cause serious bodily harm. Use a water-dampened cloth in order to remove any dust on brake parts. Equipment is available commercially in order to perform this washing function. These wet methods prevent fibers from becoming airborne.
  1. With the tire and wheel assemblies removed and the brake rotors retained by wheel lug nuts, clean the brake pad lining contact surfaces of the brake rotor with denatured alcohol or an equivalent approved brake cleaner.
  2. Inspect the braking surfaces of the brake rotor for the following braking surface conditions:
    • Heavy rust and/or pitting. Light surface rust can be removed with an abrasive disc; heavy surface rust and/or pitting must be removed by refinishing the rotor. See BRAKE ROTOR REFINISHING  under SERVICING.
    • Cracks and/or heat spots.
    • Excessive blueing discoloration.
  3. If the braking surfaces of the brake rotor exhibits one or more of the braking surface conditions, refinish or replace the brake rotor.
  4. Using a calibrated micrometer, measure and record the thickness variation of the brake rotor at 4 or more points, around the rotor. See Figure. Ensure that the measurements are only taken within the brake pad lining contact area and that the micrometer is positioned the same distance from the outside edge of the rotor for each measurement.
  5. Compare the thickness variation recorded to the brake rotor maximum allowable thickness variation specification. See DISC BRAKE SPECIFICATIONS .
  6. If the brake rotor thickness variation exceeds the specification, the rotor requires refinishing or replacement.
  7. Using the micrometer, measure and record any grooves present on the rotor braking surface.
  8. Compare the scoring depth recorded to the brake rotor maximum allowable scoring specification. See DISC BRAKE SPECIFICATIONS .
  9. If the brake rotor scoring depth exceeds the specification, or if an excessive amount of scoring is present, the rotor requires refinishing or replacement.
  10. Mount a Dial Indicator (J-8001) or equivalent, and position the indicator button so it contacts the brake rotor at a 90 degree angle, approximately 0.5" (13 mm) from the rotor's outer edge. See Fig 1.
  11. Measure and record the lateral runout of the brake rotor.
    1. Rotate the rotor until the lowest reading is displayed on the indicator dial, then zero the dial.
    2. Rotate the rotor until the highest reading is displayed on the dial.
    3. Measure and record the amount of sealed wheel bearing play, then subtract the bearing play from the lateral runout recorded to obtain the true amount of lateral runout.
  12. Compare the lateral runout of the brake rotor to the following specification: Brake rotor maximum allowable lateral runout 0.002" (0.06 mm) on front brakes.
  13. If the brake rotor lateral runout exceeds the specification, the rotor requires refinishing or replacement.
Fig 1: Measuring Rotor Runout
G00310120Courtesy of GENERAL MOTORS CORP.
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