Using a micrometer, measure the disc thickness in at least eight positions, approximately 45 degrees apart and 0.39 in. (10mm) in from the outer edge of the disc. The minimum thickness is 0.87 in. (22mm), with a maximum thickness variation of 0.0006 in. (0.015mm).
If the disc is below 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.