Measuring Brake Rotors
Take lateral runout and parallelism measurements before discarding or machining a brake rotor. In time and from excessive heat, a brake rotor warps and becomes distorted, causing vibration while braking. Use a dial gauge to measure a rotor’s lateral runout. Use an outside micrometer to measure a rotor for thickness variation or parallelism.
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, which is harder than the surface of the rotor and left behind when machining. Replace a rotor with cementite deposits.
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.