Brake Drum Inspection and Measurements
Brake drums are cast iron or iron lined aluminum. The cast iron inner linings are harder and more brittle than the area around the hub and lug holes. It is not recommended to strike the outer area of the drum where the shoes ride. Also, a cast iron 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 usually 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 these measurements. Compare them to the diameter stamped on the outside 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 and use the highest or largest measurement retrieved. Compare it to the MAX "maximum diameter" stamped on the outside of the drum.