Parallelism

Parallelism is the variation in the thickness of the disc brake rotor. If the two rubbing surfaces of the disc brake rotor are not parallel, the disc brake rotor may cause excessive brake pedal travel, a pulsating brake pedal, or noise.
Two methods can be used to check if the two faces of a disc brake rotor are parallel. A micrometer can be used to measure the disc brake rotor thickness at 12 points approximately 30 degrees apart and 25.4 mm (1 inch) from the outer edge of the disc brake rotor.
The other method is to measure the disc brake rotor on a precision lathe designed for machining disc brake rotors. Attach two dial indicators, one on each side of the disc brake rotor, so that the stylus of each indicator contacts the rubbing surface, directly opposite each other, approximately 9.5 mm (.38 inch) from the outer edge of the disc brake rotor.
Zero both indicators and rotate the disc brake rotor while watching both dials. If the total readings of both indicators exceed the specified limit for parallelism (0.010 mm/0.0004 inch), the disc brake rotor must be refinished or replaced.