For 1990-2009 cars

Check thickness variation by measuring the rotor thickness at four or more points around the rotor's circumference. Use a micrometer calibrated in ten-thousandths of an inch, and make all measurements at the same distance in from the edge of the rotor.

A rotor which varies in thickness by more than 0.025 mm (0.001 inch) can cause pedal pulsation and/or front end vibration during braking, and thus should be refinished to specifications, or replaced.