Brake Rotor/Disc: Testing and Inspection

ROTOR TOLERANCE AND SURFACE FINISH
During manufacture of the brake rotor, tolerances of the braking surfaces for flatness, parallelism and lateral runout are held very close. The maintenance of close tolerances on the shape of the braking surfaces is necessary to prevent brake roughness or pulsation. In addition to these tolerances, the surface finish must be held to a specified range, 60 Ra roughness or less. A new rotor falls into this surface finish range, as will a rotor which is refinished exactly. Control of the braking surface finish is necessary to avoid problems of hard pedal apply, excessive brake fade, pulls and erratic performance. In addition, control of the surface finish can improve lining life. Light scoring of the rotor surfaces not exceeding a depth of 1.5 mm (0.060 inch) which may result from normal use, is not detrimental to brake operation.

THICKNESS VARIATION CHECK
Check thickness variation by measuring the rotor thickness at four or more points around the rotor's circumference. Use a micrometer calibrated in ten-thousandths of an inch, and make all measurements at the same distance in from the edge of the rotor. A rotor which varies in thickness by more than 0.013 mm (0.0005 inch) can cause pedal pulsation and/or front end vibration during braking, and thus should be refinished to specifications, or replaced.

LATERAL RUNOUT CHECK
The best way to check lateral runout is with the wheels still installed on the vehicle. This gives a more accurate reading of the total indicated runout (TIR) under actual braking conditions. If you do not have the equipment to do the check with the wheels installed, the next best reading can be made with the wheels removed but with the caliper still installed.

NOTE: If you must remove the wheel, re-install the wheel nuts to retain the rotor.

1. Clean rotor surface.
2. Fasten a dial indicator to the steering knuckle so indicator button contacts the rotor surface about 13 mm (1/2 inch) from the outer edge.
3. Zero the dial indicator.
4. Move wheel (or rotor) one complete revolution, and check the TIR. TIR must not exceed 0.08 mm (0.003 inch).
- In some cases, excessive lateral runout can be improved by indexing the rotor on the hub one or two bolt positions from the original position.
- If the lateral runout cannot be corrected by indexing, check the hub and bearing for excessive lateral runout, or looseness. If the hub and bearing lateral runout exceeds 0.040 mm (0.0015 inch), replace the hub and bearing.
- If lateral runout is within specifications, refinish or replace the rotor, as needed.

NOTE: Whenever the brake rotor has been separated from the wheel bearing flange, clean any rust or foreign material from the mating surface of the rotor and flange. Failure to do this may result in increased lateral runout of the rotor, and brake pulsation.

5. If lateral runout exceeds the amount specified in Step 4, refinish or replace the rotor, as needed.