W108/W111 upper a-arm
 

My right upper a-arm has some play in the inner pivots. New replacement parts are ~$450, so I thought I'd try my luck with a used arm from eBay. It has less play than my arm, but still has some on the front side.
I decided to take the pivot off to see how it works. It is weird that it threads onto the a-arm and the pivot shaft at the same time... When I screwed it back in place it tightened on the pivot shaft, and now it does not rotate with the arm, like it should. Do I need to undo it and screw it back with the arm at a different angle? I want to figure out how this goes back together before I tackle the one on the car.

When it is assembled you can rotate the pivot and it will move right or left as it threads each direction. It can be turned until it stops on the seal. Logic would dictate it should be assembled in the center position so it does not change the castor. But I guess you could do that to add more castor. I have taken mine apart and did not have any issues so not sure what is going on.

Sounds like you assembled it wrong. The "nut" should screw into the A-arm and tighten in that but should not tighten against the pivot pin. I'm guessing that when you put it together the arm was not centered on the pivot shaft and the nut bottomed out on the pivot before tightening in the A-arm. The "nut" pivots on the threaded pivot pin, and though it is a thread and will move side to side as it pivots, the little amount is does pivot will not affect the alignment significantly.

The Mercedes of the 1960's used this design. I have also seen it on some American cars of this era. This design requires regular greasing or the threads will lock up, causing the nut to loosen in the A-arm and wearing the arm. The nut needs to stay tight in the arm.

Thanks guys! It may be a while before I have time to mess with it again, but it makes sense.