Brake Rotor/Disc: Testing and Inspection

ROTORS

DIAGNOSIS AND TESTING
The rotor braking surfaces should not be refinished unless necessary.

Light surface rust and scale can be removed with a lathe equipped with dual sanding discs. The rotor surfaces can be restored by machining in a disc brake lathe if surface scoring and wear are light.

Replace the rotor under the following conditions:
^ severely scored
^ tapered
^ hard spots
^ cracked
^ below minimum thickness

ROTOR MINIMUM THICKNESS
Measure rotor thickness at the center of the brake shoe contact surface. Replace the rotor if worn below minimum thickness, or if machining would reduce thickness below the allowable minimum.

Rotor minimum thickness is usually specified on the rotor hub. The specification is either stamped or cast into the hub surface.

ROTOR RUNOUT





Check rotor lateral runout with dial indicator C-3339 (1) (Fig. 32). Excessive lateral runout will cause brake pedal pulsation and rapid, uneven wear of the brake shoes. Position the dial indicator plunger approximately 25.4 mm (1 inch) inward from the rotor edge. The dial indicator should be positioned in the center of the rotor surface. Maximum allowable rotor runout is 0.102 mm (0.004 inch).

ROTOR THICKNESS VARIATION
Variations in rotor thickness will cause pedal pulsation, noise and shudder.





Measure rotor thickness at 6 to 12 points around the rotor face (Fig. 33).

Position the micrometer (1) approximately 25.4 mm (1 inch) from the rotor (2) outer circumference for each measurement.

Thickness should not vary by more than 0.013 mm (0.0005 inch) from point-to-point on the rotor. Machine or replace the rotor if necessary.

BRAKE DRUM IN HAT ROTOR
The maximum allowable diameter of the drum braking surface is indicated on the drum outer edge. Always replace the drum if machining would cause drum diameter to exceed the size limit indicated on the drum in hat.

BRAKE DRUM RUNOUT
Measure drum diameter and runout with an accurate gauge. The most accurate method of measurement involves mounting the drum in a brake lathe and checking variation and runout with a dial indicator.

Machine the drum if runout or variation exceed values. Replace the drum in hat rotor if machining causes the drum in hat rotor to exceed the maximum allowable diameter.