Originally Posted by
Big R
Bolt side is fine...and, yes, it's a 5mm allen
It's the nut side that I'm having an issue with as hanger is not threaded...nut is similar to nuts on chainring.
It sounds like the hanger was repaired in the past by drilling out the original threads and adding a threaded insert.
As recommended, a putty knife or other wide piece of metal can be used as a holder to keep the insert from turning. Another possibility is to remove the derailleur, remove the insert and degrease the back of the hanger and shoulder of the insert. Then it may stay in place as you tighten the derailleur's attachment bolt.