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Brake Disc Thickness Variation Measurement

Warning: Refer to Brake Dust Warning in the Preface section.

Note: Any disc brake disc that exhibits thickness variation exceeding the maximum acceptable level must be refinished or replaced. Thickness variation exceeding the maximum acceptable level can cause brake pulsation.

1. If the inboard friction surface of the brake disc is not accessible, reposition and support the calliper with the brake pads. Refer to Front Disc Brake Pads Replacement and/or Rear Disc Brake Pads Replacement .
2. Clean the friction surfaces of the brake disc with methylated spirit, or an equivalent approved brake cleaner.



3. Using a micrometer calibrated in thousandths-of-a-millimeter, or ten-thousandths-of-an-inch, measure and record the thickness of the brake disc at 4 or more points, evenly spaced around the disc.

Ensure that the measurements are only taken within the friction surfaces and that the micrometer is positioned the same distance from the outer edge of the disc, about 13 mm (½ in), for each measurement.

4. Calculate the difference between the highest and lowest thickness measurements recorded to obtain the amount of thickness variation.
5. Compare the thickness variation measurement to the following specification:

Specification
Brake disc maximum allowable thickness variation: 0.025 mm (0.001 in)

Note: Whenever a brake disc is refinished or replaced, the assembled lateral runout (LRO) of the disc must be measured to ensure optimum performance of the disc brakes.

6. If the brake disc thickness variation measurement exceeds the specification, the disc requires refinishing or replacement.