Bells are the mouth of the instrument and have an influence on sound, intonation and tone stability in the high register. They are available in many different shapes, forms, materials, bores, thicknesses, and with or without keys. Since bores can change over time it is sometimes advisable to rotate the bell to see if it sounds better in a position other than label-up.
The bell's length and opening diameter have a big influence on intonation, especially on low E/long B. Choosing a shorter bell can help if these notes are too flat.
Bells can also facilitate an extra low E resonance key. This solves the well-known low E pitch problem in many cases and makes the sound of the low E fit much better into the scale. It will require that you also have the low e mechanism on your instrument.
Most bells have a metal ring at the top as reinforcement against cracking during assembly. On new instruments, it is not uncommon to encounter problems fitting the bell all the way onto the lower joint. This is because the clarinet's lower tenon can expand when humidified but the bell's metal ring prevents expansion to the same degree. If this occurs, follow the instructions already given for barrels.
The other ring found on most bells is the larger, lower ring. It often has a dull finish and is not silver-plated. Because the ring was shrunk and pressed onto the bell when produced, any sliver plating would be damaged in the process. Both upper and lower rings are very good protection against accidental damage.
Loose ring problems
When the humidity gets very low, rings often become loose and can buzz or rattle when playing. A quick and easy solution is to put a wet cloth around the ring or to ask your repairman to shrink the ring so that it fits again. If rings on a barrel or top of the bell become loose enough to be completely removed, a thin layer of paper can be placed around the wood and the ring pressed over it. Never glue any of the rings.