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If you go to mouthpiece express, there are some mouthpiece dimension charts there. Also, Custom Music has dimensions for all PT mpcs. These may give you reference points for what you're playing now and let you find something similar.

I don't personally like the wider rim of the 24AW (the narrower mpc that I own). I tend to think of the Bach 18 and Bach 12 as more or less "average rims" -- not too wide, not too narrow, not too "sharp" of an inside rim edge. I actually like a little sharper edge and use the standard Conn Helleberg a lot. The Bach 12 is about the same cup diameter as the Helleberg, has a "gentler" inner rim edge, and I suspect isn't quite as deep. A decent, medium-large mouthpiece that my ear thinks provides a little brighter sound than the Helleberg.

I used a Helleberg with my 186 CC, exclusively, and liked the result. I suspect a Bach 12 would also work well.

Regards,

The Bach 12 is one of the mouthpieces that is on my list of favorites. As a matter of fact, I am experimenting with trying the 12 for big bass stuff and the 22 for higher lyrical stuff like Bordogni. We'll see how it works out.

For sheer low-end power, I think the 12 is out-powered by the Yamaha Jim Self, Miraphone TU33, and maybe the Kelly 18. Although the internal diameter of the 12 feels similar to these mouthpieces, I would guess stuff like throat diameter and general tuning on the other mouthpieces is more to the low end than the 12 (or even the 7).

While I can do OK on the 22 and 12, I have never been able to more than a mediocre job (if even that) on the 18.

Remember that Bach are notorious for not cutting their mouthpieces to "spec," and that there about as many variations as to cup diameter and profile in a Bach as there are in the copies and facsimiles. Roughly, and I mean very roughly, a Bach cup will be a tad larger than its "factory spec." This applies to all their mouthpieces, not just the tuba mouthpieces. In addition to my 18, which is @ 1.28 instead of the "factory" 1.26, I have a 3C trumpet mouthpiece that is almost as large as a lot of 1 1/2 C mouthpieces, and a Mt. Vernon 7 that is also larger. There are numerous threads discussing the inconsistencies on other forums, especially trumpet forums.