Checking Front Brake Discs (Evaluating Cracks)

Different vehicle types have different brake disc sizes. This produces differences for the maximum crack length limits.

The maximum crack length limit is calculated as follows:

0.5 x a (-a-  = width of friction surface in mm).

Fig 1: Identifying Friction Surface Width

Courtesy of PORSCHE CARS NORTH AMERICA, INC.

The length of a crack can be measured with a steel rule, for example. -steel rule- 

Fig 2: Measuring Brake Discs Crack Length Using Steel Rule

Courtesy of PORSCHE CARS NORTH AMERICA, INC.

Information 

Irrespective of the crack length, the brake disc must be replaced if the crack has extended as far as the outer and/or inner edge of the disc.

As a precaution, brake discs must be replaced when:

• the → Maximum crack length: 0.5 x a mm  (a = friction surface width in mm) is exceeded!
• the → Maximum crack width: 1 mm  is exceeded!
• the → Maximum crack depth: 1.5 mm  is exceeded!