How to Check Brake Rotors With a Magnetic Dial Indicator

The disk brakes on today's automobiles are very effective and reliable. From time to time, however, problems can occur. One such problem is a brake judder. This is a vibration in the brakes that is caused by a wobble in the brake rotor. This wobble, in turn, can have three possible causes. Either the disk is warped, the disk is not installed straight or there is thickness variation in the disk. The first two causes can be diagnosed by measuring the disk run-out, while thickness variation is determined with a brake disk micrometer.

Instructions

1

Park the vehicle. Do not set the parking brake. Leave the transmission in neutral. If working on a front brake, securely block the rear wheels to prevent vehicle movement. Securely block the front wheels if working on a rear brake. Partially loosen the wheel lug nuts about one turn each and then jack up the vehicle. Place the vehicle securely on an axle stand. Finish removing the lug nuts and pull off the wheel.

2

Mark the disk at eight equally spaced points around its rim and number the points from one to eight.

3

Measure the thickness of the disk at each of the marked points, about 1/2 to 3/4 inches in from the disk rim. Record the measurements.

4

Set the dial indicator stand close to the front face of the brake disk. Then affix the dial indicator to the stand such that the indicator probe touches the disk at a point about 1/2 to 3/4 inches in from the rim. The probe should be perpendicular to the disk surface. Rotate the disk by hand until the indicator probe is at the first mark. Zero the indicator and record the measurement for the first mark as zero. Rotate the disk by hand until the probe is at the second mark and record the indicator reading. Continue until you have readings for all the marks. These are your disk run-out measurements.

5

Compare your readings to the manufacturer's specifications. The minimum allowable disk thickness is often stamped into the disk, either near the middle or somewhere around the rim. Otherwise, check the vehicle's shop manual or aftermarket repair manual. These are often available at the local public library. Typically, the difference between the maximum and minimum disk thickness measurements should be 0.008 inches at most and the smallest measurement should be above the allowable minimum thickness value. The difference between the maximum and minimum disk run-out measurements should be less than 0.002 inches for passenger vehicles and 0.003 inches for light trucks and large SUVs.