I think it’s the pivot bolt anyway… The bit that attaches to the derailleur hanger…
I have a 9 spd Saint rear derailleur and DMR Chain tugs / mech hanger and it doesn’t seem to thread in and I might have fucked the thread a bit now. Do derailleurs have different sized bolts? That’d be annoying. Holla @ me. Thanks.
m810?
long story short, derailleurs do have different bolts, but theyre all the same thread pitch. it’s more likely that you’ve fucked the hanger rather than the bolt in the mech, mtb stuff use hard steel.
tl:dr i might have something anyway.
They’re all 10mm x 1mm.
If you’ve stuffed the threads up on the frame either replace the hanger or try to re-tap it. Short of a proper tap you can use most any 10x1mm rear track axle. Start from the inside of the hanger tab (where the hub sits) and work your way to the outside. Slowly and with lots of cutting fluid or some kind of thin lubricant. Go bit by bit, take it out clean the threads, Start again. It needs some force but not a lot, or else you might stuff things up even more.
Good luck 
Spizza to the rescue! Solved! Thanks a bunch xoxo