As a general rule of thumb, passenger car brake drums can be turned .060" oversize safely, and most 1/2 ton truck drums will handle .090" over without a problem. If you're dealing with 3/4 ton or larger stovebolt drums, don't machine them at all, as they're virtually impossible to replace. New ones don't exist and "good used" drums command a king's ransom if they're available at all.
Jerry
