well you didn't say what your yamaha mouthpiece is, so i can't tell you want it would be equivelent to in bach.

i have two questions for you;

Why do you want to switch from yamaha to bach?

Does the mouthpiece you have right now not working for you?

if the piece you are using right now is not working for you, ask yourself does it feel too small, or too big? if you mouthpiece feels just fine, then stick with it. don't just switch to bach because you think it is better. i use a bach strad trumpet but i use a yamaha mouthpiece. if the mouthhpiece you are using right now is not working for you then i suggest you try a bunch of diferent brands and sizes of mouthpieces before buying any.

sorry for the confusion, i have tried several mouthpieces, and have narrowed it down, and i just need the yahama i have in bach like i have stated. and i have nothing against yamaha mouthpieces.. i have a stomvi trumpet, not one to care about brands, i just need to know how to figure out what my mouthpiece is in bach, i can do it myself i just need to know how. thanks.

why do you want to find out what your yamaha would be in bach?
if you just really want to know there are mouthpiece comparison charts on the internet. For example the mouthpiece i use is a yamaha 9C4 which is equivelent to the bach 5C.

here is a chart
http://www.ibowtie.com/tmptmpccharts.html

there is no way to just figure out what a mouthpiece would be in bach, different companies use completlely different numbering and labeling systems

thank you for the chart,
that's exactally what i needed.!
thanks.