2. Checking brake discs for minimum thickness

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In the presence of high pressure forces, at the innermost and outermost hole-free friction plate track -arrows- , the brake pad friction surface is the least worn, compared to the middle holed area. Refer to Fig 1 . As a result, in the case of weaker braking, a relatively higher amount of pressing on the surface arises in these areas and therefore greater wear of the friction plates. The natural balance between stronger and weaker braking ensures that in almost all cases the typical wear profile of a perforated friction plate arises (inner and outer smooth friction rim area) is the most worn -arrows-. Refer to Fig 1 .
Arrows Zone where most wear occurs in brake disc friction surface.

Fig 1: Identifying Brake Disc Friction Zones
Courtesy of PORSCHE OF NORTH AMERICA, INC.
NOTE:
- As opposed to smooth brake discs, by which the minimum thickness is measured in the middle of the disc (effective friction radius), in the case of perforated brake discs, the minimum thickness must be measured on the inner or outer friction surface track, -arrows-, whichever is most worn. Refer to Fig 2 .
  
  Fig 2: Measuring Points For Checking Minimum Thickness Of Brake Discs
  Courtesy of PORSCHE OF NORTH AMERICA, INC.
- Brake disc thickness/wear limit: Max. 2.0 mm total wear permissible. Refer to the technical data for the new dimension and other dimensions Refer to TECHNICAL DATA/BRAKE WEAR LIMIT - 911 CARRERA , TECHNICAL DATA/BRAKE WEAR LIMIT - GT3 , TECHNICAL DATA/WEAR LIMIT BRAKE - GT3 RS AND GT3 AS OF MODEL YEAR 2004 , TECHNICAL DATA/BRAKE WEAR LIMIT - 911 CARRERA 4 , TECHNICAL DATA/BRAKE WEAR LIMIT - 911 CARRERA 4S or TECHNICAL DATA/BRAKE WEAR LIMIT - 911 CARRERA 4S WITH PCCB .
- Measure the brake disc thinnest point with a suitable micrometer screw, at one of the two smooth friction rim zones -arrows- (at the most worn friction rim area). See Fig 3 .

Example of wear

Fig 3: Identifying Smooth Friction Rim Zones
Courtesy of PORSCHE OF NORTH AMERICA, INC.

The drawing shows the typical grooved wear profile -3 arrows- of a perforated grey cast iron brake disc, which has been used to the wear limit, under tough conditions i.e. constant operation mode (test operation mode).