For 1990-2009 cars

Thickness variation can be checked by measuring the thickness of the rotor at four or more points around the circumference of the rotor. All measurements must be made at the same distance in from the edge of the rotor. Use a micrometer graduated in ten-thousandths of an inch (0.0001 in), or a micrometer graduated in 0.0025 mm.

A rotor that varies in thickness by more than 0.013 mm (0.0005 in) can cause pedal pulsation and/or front end vibration during brake applications. A rotor that does not meet these specifications should be refinished to specifications or replaced.