Originally Posted by
gaucho777
You can try to chase the threads by installing the derailleur mounting bolt from the back side of the hanger. It should thread all the way through and fix any damaged or crossed threads. Clean up the threads well, and apply liberal amounts of oil (cutting oil, ideally, but standard oil is better than nothing). Use a cheap spare derailleur mounting bolt if you have one.
This ^
Not removing any metal this way similar to a roll form tap, a superior tapping method in almost every circumstance.