Maintanence Manual procedure 4251. Some excerpts:
Check brake disks for score marks and cracks. Cracks in a circumferential direction up to 0.5mm in depth are permitted.
Ventilated brake disks with hairline cracks up to 25 mm in length, which may show up under high loads, need not be replaced. Replace disks with open cracks and score marks deeper than 0.5 mm immediately.
Ventilated front brake disks - 19.4 mm
Solid front brake disks - 9 or 10 mm
Rear brake disks - 7.3 mm
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino