A brake drum micrometer is an instrument for measuring accurately the diameter of a brake drum. A brake drum micrometer should be used any time the brake drums are removed for servicing. The braking surface diameter should be checked at several points to determine if it is within the safe oversize limit stamped on the outer surface of the brake drum. If the braking surface diameter exceeds specs then the drum needs replacing. If the braking surface diameter is within specs, the drums should be cleaned and inspected for scores, grooves, cracks, taper, out of round, and heat spotting.