Dura Ace 7700 rear derailleur will not fit into hanger I am in the process of building up a Cannondale CAAD4 Saeco team frame using Dura Ace 7700 components from the same era of the frame. I am learning as I go along, so I apologize if these questions sound stupid. I have found a Dura Ace 7700 rear derailleur short arm but it does not fit into the hanger tab that is currently on the bike. Is there a way to change the screw on the derailleur itself or replace the hanger tab? (It looks like it is an added component).
Practically all derailleur hanger bolts are M10x1.0mm, so it's strange that the derailleur bolt will not screw into the hanger.

Possible reasons:


The thread in the hanger is damaged
The bolt thread is damaged (but from the pic it looks OK)
It's a hanger for a Campagnolo derailleur which uses M10x25TPI - but that's very close to M10x1.0mm, you should at least be able to start the bolt in the thread.


Do you have another bike to try the derailleur bolt in?  Perhaps obtain a M10x1.0mm bolt from a hardware store and see if it fits the hanger.

Googling for 'replacement derailleur hanger' should get you some results. Currently Wheels Manufacturing offers a wide range of replacement hangers and provides a search by bike brand. Start here.