The runout of a disk brake rotor is a measure of how much the rotor wobbles as it turns. Runout is measured by placing the probe of a dial indicator against the rotor and taking readings at a minimum of eight points around the rim of the rotor as the rotor is rotated by hand. The difference between the highest and lowest dial indicator readings is the runout. Rotor thickness measurements should be done in conjunction with runout measurements as variations in rotor thickness will also be read as runout, and doing runout measurements alone can lead to an incorrect diagnosis.
The maximum disk brake rotor runout for passenger vehicles, including cars, compact SUVs and compact pickup trucks is normally about 0.002 inches. Some manufacturers set their specifications as low as 0.001 inches.
The maximum disk brake rotor runout for full sized SUVs and pickup trucks is normally about 0.003 inches with some manufacturers setting their specifications as low as 0.0015 inches.
Most manufacturers specify a maximum allowable variation in rotor thickness measurements of 0.0002 to 0.0005 inches.
