TubeNet

Where is the bore size of a tuba generally measured?
What were the bore sizes of the old Conn 2J and 3J horns?

Bored is usually measured in the valve section. The tubing is cylindrical in this section and then gets larger(conical)after the valves. Sometimes the 4th or 5th valve will have a larger bore. On Conn 2J and 3J I think the bore is .656 since I think they are generally the same frame, just different valve configurations, as the 11J/12J series and Conn used the same tubing stock on this whole line.

To be a little more specific, bore is typically measured at the inner diameter of the second valve inner slide.