Q:

How can I tighten a loose rear derailleur onto the derailleur hanger?

I have recently tried to install an NX Eagle derailleur on my bike, but I could not tighten it onto the hanger tight enough to firmly fix it, so that it does not rotate around the pivot. In other words, it would just turn around the fixing bolt like it were an axle, no matter how tight I screwed it in. The tightening torque definitely is not the problem.
The only way I could solve this was by putting a pedal washer (because I had one nearby that I did not need) under the fixing bolt and then tightening. This works, but it does not seem like a great solution, so I would like to know if somebody had a similar issue, what is the cause (maybe the bolt thread is too short), and is there a better way to solve it?
Is there a dedicated washer for this purpose? Or is the mech supposed to just turn around the pivot until it becomes tensioned by the chain, which I very much doubt?

A:

Most derailleurs are expected to rotate at the derailleur hanger bolt. This is to simplify rear wheel removal and installation as the derailleur can be moved away from the wheel path by hand. The ability to rotate is not necessarily something that affects shifting quality, but it can be undesirable, especially in certain applications.
As a very recent example, TRP announced a new downhill drivetrain in which the rear derailleur has a separate lever to control rotation around the bolt. To quote a news site:

Named after Aarons mechanic John Hall, the Hall Lock is, in essence, a lever integrated into the derailleur mount which locks the movement of the B-knuckle around the mounting bolt when closed. Locking the B-knuckle is claimed to reduce chain noise and increase chain retention.

 

A:

After reading Grigory's explanation and other comments (thanks everyone!), I can answer my own question.
SRAM NX Eagle and other SRAM Eagle derailleurs indeed do normally rotate around the derailleur mounting bolt, so there is no need to tighten them to prevent rotation around this joint. I checked this at a bike shop that stocks Eagle-kitted bikes.
However, although this is also true for many other derailleurs, not all of them are designed to do this. For instance, my old Shimano Deore RD-M592 8/9-speed derailleur screws in tightly into the hanger and does not rotate around the mounting bolt. Instead, it has another joint a bit further back that performs this function (enabling derailleur body to rotate around axis parallel to wheel axis). I have dismounted and mounted this derailleur many times without paying much attention to this detail, which is why I assumed all derailleurs should act the same. The moral is that you should pay attention to whether your derailleur is intended to rotate around the mounting bolt, although this will probably be obvious when you screw it in (normally it will either stay loose or be fixed without any particular input from your part).
Below I attach an annotated picture of the two derailleurs/cases mentioned above, for comparison.

