Checking Grey Cast-Iron Brake Discs - Minimum Thickness

Information

Normally, brake discs are used until the maximum wear dimension is reached. → WM 4600TW TECHNICAL DATA/BRAKE WEAR LIMITS Only in a few exceptional cases is it advisable to skim the brake discs (if there is deep scoring or corrosion of the friction surface). In this case only the disc brake dressing machines approved by Porsche AG are to be used.

Only as new brake discs may be reworked!
Only rework the brake discs symmetrically, evenly from both sides!

As the pressure force of a brake piston on the brake pad varies in magnitude, different areas of wear arise on the friction surface. The brake disc can basically be divided into two areas of wear: outer wear areas and the central wear area.

The natural balance between stronger and weaker braking ensures that in almost all cases the typical wear profile of a perforated (cross-drilled) brake disc (friction plate) arises.

This means that the inner and outer smooth friction edge area is most worn.

Fig 1: Locating Outer Wear Areas Brake Disc Thickness

Courtesy of PORSCHE CARS NORTH AMERICA, INC.

Fig 2: Locating Central Wear Areas Brake Disc Thickness

Courtesy of PORSCHE CARS NORTH AMERICA, INC.

Measuring brake disc thickness
Information

Unlike smooth brake discs, on which the minimum thickness is measured in the middle of the disc, the minimum thickness of perforated (cross-drilled) brake discs must always be measured on the inner and outer friction surface track that is most worn -arrows- .
1. Central wear area
2. Outer and inner wear area
  
  Fig 3: Locating Brake Disc Thickness Wear Areas A And B
  Courtesy of PORSCHE CARS NORTH AMERICA, INC.
Measure the brake disc thickness -X- in the area of the outer or inner friction surface track -arrows- at least three different points (-A, B, C- ) offset by 120°.

To do this, use a suitable micrometer or a brake disc gauge.

Fig 4: Measuring Brake Disc Thickness
Courtesy of PORSCHE CARS NORTH AMERICA, INC.