Using a micrometer, measure the disc thickness at eight positions, approximately 45 degrees apart and 0.39 in. (10mm) in from the outer edge of the disc. The minimum thickness is 0.882 in. (22.4mm) for front rotors or 0.331 in. (8.4mm), with a maximum thickness variation of 0.0006 in. (0.015mm).
If the disc is beyond limits for thickness, remove it and install a new one. If the thickness variation exceeds the specifications, replace the disc or turn rotor with on the car type brake lathe.