Checking Grey Cast-Iron Brake Discs - Evaluating Cracks

No need to replace brake disc as the perforation crack length is small (less than 5 mm). 

Fig 1: Perforation Crack Small Length (Less Than 5 Mm)

Courtesy of PORSCHE CARS NORTH AMERICA, INC.

Some perforation cracks are over 7 mm long.

Brake disc must be replaced (maximum permitted perforation crack length is 5 mm)! 

Fig 2: Identifying Perforation Big Crack Length (Greater Than 5 Mm)

Courtesy of PORSCHE CARS NORTH AMERICA, INC.

Brake disc with cracked friction plate rim -arrow-  . Brake disc must be replaced! 

Fig 3: Locating Friction Plate Rim Damaged Area

Courtesy of PORSCHE CARS NORTH AMERICA, INC.