One of the most frustrating ordeals lately has been trying to find the “discard” or “machine to” spec on an aftermarket rotor that has seen 40,000 miles. Most OE rotors have have the spec cast or stamped on the rotor’s hat or bell.
On most aftermarket rotors the specifications are etched or laser inscribed on the edge of the plates. I hate to complain, but most of the markings are gone before the pads are worn.
So where do you find the discard specifications if they are gone? AllData, Mitchell and the OE service information have the specifications including runout and disc thickness variation. But, there is another source for the for just the discard specification. Most rotor manufacturers include the discard specification in their electronic catalogs.