Brake Disc Service

Servicing of disc brakes is extremely critical due to the close tolerances required in machining the brake disc to insure proper brake operation.
The maintenance of these close controls of the shape of the rubbing surfaces is necessary to prevent brake roughness. In addition, the surface finish must be non-directional and maintained at a micro inch finish. This close control of the rubbing surface finish is necessary to avoid pulls and erratic performance, and to promote long lining life and equal lining wear of both left and right brakes.
In light of the foregoing remarks, refinishing of the rubbing surfaces should not be attempted unless precision equipment, capable of measuring in micro inches (millionths of an inch) is available.

Fig. 1 Checking rotor for lateral runout:

Fig. 2 Checking rotor parallelism (thickness variation):

To check lateral runout of a disc, mount a dial indicator on a convenient part (steering knuckle, tie rod, disc brake caliper housing) so that the plunger of the dial indicator contacts the disc at a point one inch from the outer edge, Fig. 1. If the total indicated runout exceeds specifications, install a new disc.
To check parallelism (thickness variation), mount dial indicators, Fig. 2, so the plunger contacts rotor approximately 1 inch from outer edge. If parallelism exceeds specifications, replace rotor.