I have a bike with the Athena 11 speed triple group, and I need to remove the rear derailleur. I can't undo the derailleur hangar bolt. It does not engage a 4, 5, or 6 mm Allen key. I looked on the Campy site and found the installation information, and it says the bolt should have a Number 25 Torx socket. I have a 27, which does not fit. I bought a Torx 25, and it does not grab. Looking inside the socket with a light, there is no sign of stripping or other damage in the socket.

Is it possible that Campy changed their tune? The derailleur is about 2015 or 2016 based on recollection; it could have been NOS.

The reason to get it off is to check whether the derailleur hanger is out of alignment. It's my wife's bike and she dropped it, and since then has been complaining of gear misbehavior - noises, hard to shift, et cetera.

Any case, are there any odd bolts that are known to be able to fit inside the mech, fit the thread, and not fit a T25? I'm thinking a T30 (and I'll buy a T30 socket now), but if that attempt fails, I will be out of reasonable options.

The bike was assembled by an LBS that I thought was pretty good, but now ... I would not go in there again. They are the only ones who should have installed the rear mech, and I did not R&R it in the past two years, i.e. since delivery as a finished bicycle.