thanks for that, while a tap is obviously better, the specific bottom bracket thread ones are over £50 for ones I've seen whereas undamaged steel loose cups are as cheap as chips or at least they were until this gets out...............
Used this method to clean out a very clogged up thread on an aluminium frame only last week.
Most loose bb cups will be hard which makes them even better for cleaning up threads but means that you'll probably need a grinder rather than a file to do the slots.
Still got that set you made me chris, part wound in before powder coating kept the threads from getting gummed up and cleaned them up a treat when coming out. The coaters asked if they could keep them I said no .