The thickness of the discs must be checked with a micrometer by measuring the thickness of the disc at four or more points along the circumference of the disc. All measurements should be taken at the same distance from the edge of the disc to its center (pic. 6-14).
Pic. 6-14. Measuring disc thickness with a micrometer
If at least one measuring point is less than 9.5 mm thick, the disc must be replaced.