Hy Savannah54, there are special brake drum micrometers, are we talking about drums for a light truck or for a big bolt? I think if you took your drums to a shop which does brake jobs, they would measure them for you and tell you if they are turnable or if it is time for them to be replaced. Sometimes a drum will measure as turnable, but when put on the lathe it is too much out of round to cut it back to circular without going oversize, hope that helps.
