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I have a Denis Wick 3 (small shank) mouthpiece that was once needed for a Boosey & Hawkes Eb Imperial. That horn has been replaced with a Besson 982 Eb Sovereign. I have a large shank Wick 3L for the Sov but a now useless, expensive small shank Wick 3 sitting doing nothing. I have searched the internet for a suitable adapter to convert the small shank 3 to a large shank 3L, but I'm not getting any information that gives me confidence anything offered will work. Examples: Bruno Tilz offers an adapter that converts "tuba to old German tuba", or another that adapts a F/Eb mouthpiece to "B tuba". Confusing. Can any body please direct me to an adapter that will definitely do the conversion I need?

The Tilz adapter nr. 912 tapers inside from 14,2 mm to 12,8 mm, length is 36 mm, thickness metal is 1 mm.
I do not know about the other one (F - Bb).
Best,
Hubert

Thomann has a couple of adapters on their web page. One says bass trombone to b-tuba, the other says f-tuba to b-tuba. Both have measurements of inside and outside the adapter. My guess is that the bass trombone one would be the one most likely to work as long as the b-tuba size they refer to is not Euro shank. They have a US office that could probably help.

The adapters Tilz F/Bb and Josef Klier JK nr. 250 will do the job for you. They are used to adapt a small shaft of 12,5 mm (measured at end of shaft) to a modern/regular shaft of 13,5 mm (end of shaft).
Hubert