I bought a new letterbox lock (Burgwächter ZBK-70, pdf datasheet/drawing), but the bar is too short.
I'd like to exchange that bar, but I have no clue how to loosen the bolt. So far, I tried it with telecom pliers, but it didn't work.
What is this black washer-like thing underneath the nut, by the way?
That is a circlip.
Just use a small flat screwdriver in the gap to prise it off - make sure you keep hold of it because it can ping away.
If you put the screwdriver in the gap at 3 o’clock and twist it will force the circlip over the diameter of the shaft.
What is this black washer-like thing underneath the nut, by the way?
It is a "circlip". - Operate on it under a hanky or something because they can fly pretty far when pushed out of their notch.
The "black thing" isn't a nut, there is no nut in this application.
Place a small blade screwdriver in the exposed gap between the black c-shaped clip, and give the driver a little twist. You'll see the clip starts to ease away from the rotational axis of the tumbler.
If you don't do that carefully with some covering, you can lose that clip quickly.
This "nut" cannot be tightened, but theoretically you can add some shim stock under it to tighten up the assembly.
