| Fig. 1: Drum brake service limits. The maximum inside
diameter is cast into the drum
|
After the brake drum has been removed from the vehicle, it should be inspected for run-out, severe scoring cracks, and the proper inside diameter.
Minor scores on a brake drum can be removed with fine emery cloth, provided that all grit is removed from the drum before it is installed on the vehicle.
A badly scored, rough, or out-of-round (run-out) drum can be ground or turned on a brake drum lathe. Do not remove any more material from the drum than is necessary to provide a smooth surface for the brake shoe to contact. The maximum diameter of the braking surface is shown on the inside of each brake drum. Brake drums that exceed the maximum braking surface diameter shown on the brake drum, either through wear or refinishing, must be replaced. This is because after the outside wall of the brake drum reaches a certain thickness (thinner than the original thickness) the drum loses its ability to dissipate the heat created by the friction between the brake drum and the brake shoes, when the brakes are applied. Also the brake drum will have more tendency to warp and/or crack.
The maximum braking surface diameter specification, which is shown on each drum, allows for a 0.060 in. (1.5mm) machining cut over the original nominal drum diameter plus 0.030 in. (0.76mm) additional wear before reaching the diameter where the drum must be discarded. Use a brake drum micrometer to measure the inside diameter of the brake drums.
