Measuring Brake Rotors
Lateral runout and parallelism measurements are being discussed. The rotor will warp and become distorted causing a vibration while braking. A dial gauge is used to measure a rotor’s lateral runout. An outside micrometer is used for measuring a rotor for thickness variation or parallelism.
Measure the thickness of the rotor with an outside micrometer in several places. A thick rotor is capable of absorbing and storing more heat than a thinner one. When a hot rotor cools quickly, like when sitting in a deep puddle too long, the rotor may not return to this original form and begin to distort or warp. This is especially true with single bladed non-vented rotors found on the rear brake units of many vehicles today. They can also develop dark pad deposits called cementite, that are harder than the surface of the rotor and left behind when machining. A rotor with cementite deposits should be replaced.