Brake Drum Surface and Wear Inspection

Special Tools 

CH-8001  Dial Indicator Set

For equivalent regional tools, refer to Special Tools .

1. With the brake drum removed, clean the brake shoe lining contact surface of the brake drum with denatured alcohol or an equivalent approved brake cleaner.
   WARNING: Refer to Brake Dust Warning .
2. Inspect the braking surface of the brake drum 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 drum.

   • Cracks and/or heat spots
   • Excessive blueing discoloration
   • Missing balance weights
3. If the braking surface of the brake drum exhibits one or more of the braking surface conditions listed, the drum requires refinishing or replacement.
4. Using a brake drum micrometer calibrated in thousandths-of-an-inch, measure and record any grooves present on the drum braking surface.
   Fig 1: Drum Brake Micrometer

   Courtesy of GENERAL MOTORS COMPANY

   Ensure that the measurements are only taken within the brake shoe lining contact area.

5. Compare the groove or scoring depth recorded to the following specification:

   Specification 

   Brake drum maximum allowable scoring: 0.50 mm (0.020 in)

6. If the brake drum scoring depth exceeds the specification or if an excessive amount of scoring is present, the drum requires refinishing or replacement.
7. Mount the brake drum on a brake lathe.
8. Mount a dial indicator, CH-8001  indicator (1) or equivalent and position the indicator button so it contacts the braking surface of the brake drum (2) at a 90 degree angle, approximately 19 mm (0.75 in) from the outer edge of the drum.
   Fig 2: Dial Indicator And Brake Drum

   Courtesy of GENERAL MOTORS COMPANY
9. Measure and record the radial runout of the brake drum.
   1. Rotate the drum until the lowest reading is displayed on the indicator dial, then zero the dial.
   2. Rotate the drum until the highest reading is displayed on the dial.
10. Compare the radial runout of the brake drum to the following specification:

    Specification 

    Brake drum maximum allowable radial runout: 0.03 mm (0.001 in)

11. If the brake drum radial runout exceeds the specification, the drum requires refinishing or replacement.