How to Read a Brake Drum Micrometer

When repairing automotive brake drums, manufacturers specify minimum tolerances, or the maximum amount of wear that a part can undergo before the part must be replaced. During the process of turning a brake drum, before the part can be reinstalled on the vehicle, the drum must be measured with a micrometer to verify the resurfaced part is still within the manufacturer's recommended tolerances.

Instructions

1

Fasten the brake drum to the brake drum lathe securely. Resurface the interior of the brake drum in successive passes until all wear lines and grooves have been removed.

2

Using the manufacturer's specification guide, identify maximum diameter allowed for the resurfaced brake drum. The measurement will be a total diameter allowed for the interior area of the brake drum.

3

Rest the brake drum on a smooth, stable work surface so the interior surface of the brake drum faces upward. Using the brake drum micrometer, measure the interior diameter of the brake drum opening. Measure this distance at multiple locations to verify that the resurfaced brake drum is a perfect circle, and not elongated, or oval shaped at any point.

4

Compare the actual measurements to the manufacturer's specifications. If the diameter of the interior opening is less than the manufacturer's tolerances, the brake drum can be safely reinstalled on the vehicle. If the brake drum interior diameter is larger than the manufacturer's tolerances, the brake drum must be replaced.