Use the information on a brake rotor thickness chart to determine the disc brake rotor minimum thickness specification for the make, model and year of your vehicle. It's important to measure the thickness of the rotors each time the brakes are serviced. If the rotor thickness is below the minimum specification, or if the rotor cannot be resurfaced without going below this dimension, it should be replaced. In certain states, the law requires replacement of the rotor.
The minimum thickness specification is an important dimension because it is the minimum dimension that the vehicle manufacturer recommends to provide safe braking. As a rotor wears it has less mass, which reduces its ability to absorb and dissipate heat. This, in turn, causes reduced braking capacity, resulting in premature fade and increased stopping distance. Wear also decreases the strength of the rotor, increasing the risk of cracking or failure.
When brake rotors are below the minimum thickness recommendation, other issues may result as well. The caliper piston may move further within the caliper body. As the piston loses support, it may jam, causing the brakes to lock. In extreme cases, the caliper piston may cease to form a seal and the system will leak hydraulic fluid, resulting in increased stopping distance or even brake system failure.