Disc Brake Rotor Inspection

The most common inspections items for rotors are wear, scoring, runout and parallelism. Excessive wear and scoring most often occur when the brakes have been operated with extremely worn or damaged pads. This damage is usually self-evident.

Runout and parallelism are more subtle patterns of wear. Runout is the degree to which the rotor runs true to the vertical axis of the hub. Parallelism is the degree to which both sides of the rotor are parallel to each other.

If runout is excessive, the rotor will push the pads away from itself and force the piston deep into its bore so that greater pedal movement will be needed to squeeze the pads against the disc during the next brake application.

To check runout, follow this procedure:

1. Adjust the wheel bearing to zero free play and zero preload.
2. Mount a dial indicator, set the gauge to zero and turn the rotor one full revolution.
3. The dial should not move more than 0.005" from zero. If it does, runout is excessive.
4. Check the wheel bearing to ensure it is not the source of the trouble. If the wheel bearing is okay, the rotor must be turned or replaced.

If rotor parallelism is excessive, there is an unacceptable variation in the thickness of the rotor. When this occurs, the brake may chatter, vibrate, or pulsate during application.

To check parallelism, measure the rotor's thickness with a micrometer at several points around the rotor. If the difference in readings exceed 0.0005", the rotor should be turned or replaced.

Refer to the brake manufacturer's specifications when deciding whether to turn or replace a rotor.