Reverb seat post help

You've likely caused an issue with the "keys" that stop the post from rotating (just little cylinders that sit in vertical grooves between the outer and inner tubes in the post). Strip down and replace the keys. It still might not fix your issue if the "grooves" are damaged though.

When you say rotates a bit, how much is a bit?

Mine is slightly warn and you do get a few degrees of rotational movement,

As FBF said it will be the keys,

OK, thanks looks like I need to get tools out and check the keys out. Its moving about 20 to 30 degrees I would say.

Oof, 20 to 30° sounds like more than damaged keys.

IIRC it is possible to unscrew the top of a Reverb if you give it a hard enough whack. Does the seat turn anticlockwise and keep going? And does it come to a stop if you turn it clockwise?

If so, just tighten it clockwise as hard as you can and keep riding, Next time the post gets stripped down, put some threadlock on the joint between the post and the seat clamp when reassembling.

That’s not how they work, the top cap just screws on and houses the upper bush, you can unscrew it completely and the post will still work, just wobbly at the top, the only parts that stop the inner shaft rotating inside the outer tube are the brass keyways, to be damaged enough to rotate, it would not go up and down.

Are you sure it’s not the whole post moving, if it’s fully inserted maybe the top cap if catching on the seat clamp and stays stationary while the post rotates.

That’s not how they work

The lower part of the seat clamp screws into the top of the post that moves up and down inside the top collar (the one with the brass pins in it). It’s normally really tight / threadlocked and doesn’t normally get dismantled. A bad fall can be enough to loosen it and the seat will turn around without the post moving. The joint can be quite difficult to see.

It’s circled in red here:

Ah gotcha. I presumed they are bonded rather than threaded.