The titanium shanks of the Boone Mouthpieces are the toughest part to make and are the reason that most companies never attempt them. They are extremely difficult to machine. We use a standard bore of .144", which is also known as a #27 bore. This is the standard for most mouthpieces.  The rim and cup part screw onto the shank and a single shank can be used for multiple cup sizes. The shank offerings can be seen and ordered from the Build Your Own page. We can turn threads on your brass mouthpiece or you can use our titanium one that matches the cup geometry.