TromboneChat

When the ring is fully tightened, the valve gets stuck. What could be the cause? Thanks for your help!

What type of Axial Flow / Thayer Valve is it? What trombone is it on? If it is binding when the ring is fully tightened, that means that something is out of alignment. Some types of AF valves have a backing plate that rests on the casing, while I think some others are kind of "floating". I could be wrong on this though.

It's an original thayer

Which kind? Stainless? Teflon core? A picture would be good.

The bearing plate is probably out of alignment, so the ring is creating a pressure point of the plate on the core when fully tightened.

Or maybe some crud built up on the plate or the core.

I agree tubing could be misaligned. Try unscrewing all the nuts that hold it together, and see if the posts line up correctly. If the screws are forcing the parts into alignment, it's putting stress on the valve casing and stop plate. Also, the ring should be snug but not overly tightened. I doubt it's something you can fix yourself. You will have to have it looked at.