Joined: Aug 2009
Posts: 7,703
Likes: 4,021
From: Berkeley, CA
Bikes: 72 Cilo Pacer, 72 Gitane GT, 72 Peugeot PX10, 73 Speedwell Ti,l, 75 Peugeot PR-10L, 80 Colnago Super, 81 Zinn, 85 ALAN Cross, 85 De Rosa Pro, 86 Look 753, 86 Look KG86, 89 Parkpre Team, 90 Parkpre Team MTB, 90 Merlin
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.