When a customer brings in a horn for repair I do two things automatically: I stick my hand inside the bell as if I’m playing it, and I grab the bell by the throat (the area near the main bell brace) and rotate my hand around it, feeling to see if it’s round.
An out-of-round bell throat can make it difficult to keep a mute installed properly, or can even make it difficult to play the horn stopped. An out of round condition can deform the ring and make it difficult or impossible to screw the bell flare on or off a detachable bell horn. Creases and dents can do the same thing.
The rim of the flare can have just as serious an effect on the throat or bell ring as a dent, crease, or out of round situation. If the rim gets bent or “potato chipped”, then it can put a tremendous force on the shape of the throat and the ring.
A customer recently asked if I could replace his ring, which was severely warped. It turned out the the creases that had gotten into the flare were the cause. Once the creases were taken out, the ring was just like new. A bad, expensive repair would have been replacing the ring. A good, low cost repair was easily accomplished by reshaping the flare properly.
Have the conditions that are making your bell come out of round taken care of by a competent repair tech. Don’t try to fix them yourself.
And never, ever push a folded rim back into place yourself. The really thin metal that makes up your bell flare can be damaged badly unless it’s fixed properly. Knowing what to fix and how is critical to a good and low cost repair.