RP 1415 - Hydraulic Brake Rotor Durability And Diagnostic Guidelines: Cracks

These are gaps in the friction surfaces that resemble small irregular grooves. Cracks are caused by the heating and cooling of the friction surfaces during the application and release of the friction materials. This situation usually indicates excessive heat or poor metallurgy. If turning the rotor does not remove the crack (maintaining the manufacturer's minimum allowable thickness or greater,) the rotor must be discarded.