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?
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.