Brake Drum: Inspection & Measurements
Brake drums are made of cast iron or cast iron lined aluminum. The cast iron inner linings are harder and more brittle than the softer metal rings area around the hub and lug holes. This is why it is not recommended to strike the outer area of the drum where the shoes ride. Also, a good drum should ring like a bell when struck. If there's a dull thunk, check for a crack in the drum's surface. Cracks are mostly too deep for machining and the drum must be discarded.
Always measure the inside of the brake drum before deciding to discard or machine on a lathe. Use a drum micrometer or an equivalent tool to retrieve the measurements, and compare them to the diameter stamped on the outside surface of the drum. If the drum is scored or grooved, place the tool into the deepest part of the groove. Take several measurements around the drum. Because the inside of a drum is being measured, use the highest or largest measurement retrieved. A 10.80" diameter is larger than a measurement of 10.55", so use the 10.80" measurement to compare to the MAX "maximum diameter" stamped on the outside of the drum.