I have been told that the rotors are manufactured with only about 40 thousandths of excess material. Several years of use and one turning would bring you right at, or under minimum thickness. Once you get that thin, heat build-up can quickly warp the rotors, giving you vibration and shake when stopping.
Once under the minimum thickness you also stand the chance of a catastrophic failure of the rotor. Example: I knew a mechanic back in Texas, who was short on funds, he needed brakes and the front rotors needed to be turned. He almost got the grooves out, but the rotors were already under minimum. Being short on cash, he used them anyways "until next payday". We had a good rain storm, he'd been driving to work, hit a Houston puddle, and the cold water making contact with the thin rotors caused a catastrophic separation of the rotor at the hub. The front tire continued down the road, while his bumper ate pavement.
Now I'll admit, I had no idea MB rotors were built to such close tolerences, but thanks to this board I do now.
73 280 SEL 4.5
[This message has been edited by MikeTangas (edited 05-18-2000).]