A drum micrometer is used to measure the inside diameter of the brake drum on vehicles with drum braking systems, primarily for rear brakes, but some antiques have four-wheel drum braking systems. Brake drums wear on the inside diameter during a brake shoe replacement when the drum is machined on a lathe. After a while, the inside diameter of the drum becomes too wide or too thin to be effectively useful. All drums for all types of vehicles have a "discard" measurement that allows the drum to be measured to this point and no further. The micrometer is used to determine how close the drum is to this discard point.
Lay the brake drum on a level surface, upside down, with the inside diameter of the drum facing upward.
Inspect the inside diameter of the drum for heat cracks or scoring. Scoring creates uneven surfaces that cut deep into the interior surface of the drum where the shoes come into contact with it. In the event of scoring, the deepest score line would need to be measured with the micrometer, as that would be the point to which you machine the drum if machining is an option.
Place the indicators of of the micrometer so they are evenly distributed on the inside of the brake drum. Apply some light, even pressure on the indicators of the micrometer and position the indicators to line up with any scores that may be present.
Holding the drum steady (it may be useful to have a helper), place your hands on each end of the micrometer and turn it in a clockwise motion around the inside circumference of the drum. Watch the dial indicator of the micrometer to measure any variance of the inside diameter. Any anomaly in measurement indicates run-out or warping of the drum and renders it useless, no matter what measurement is indicated for discard comparison.
Write down the measurement of the score that the micrometer reads. If the reading of the circumference measurement showed no variance of warping, compare the micrometer's measurement to the specifications on the brake drum/rotor discard chart.
