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What would the optimum bore size be for a rotary valve 6/4 tuba?

Is there a bore that one might call an "industry standard"?

I don't know if there is one I would consider and 'industry standard'. But... here are a few sizes that I know of:

.778 = Miraphone 186
.787 = 20mm (Amati 20.2mm)
.795 = 20.19mm
.807 (smaller Alex bore, I believe) = 20mm
.812 = 20.62mm
.827 = 21mm (Cerveny)
.827 = 21mm St Pete 201/202
.835 = 21.21mm - Miraphone 1291 BBb
.846 = 21.5 mm
.866 = 22 mm (large Rudy)

Because there is no industry standard for WHERE in the bugle the valves are placed, bore size is hard to compare like you can with trumpets.

Take two tubas that are identical except for the length of the leadpipe and the resultant placement of the valves. The same size valve will play "larger" in the shorter leadpipe, and play "smaller" in the longer leadpipe. That is one reason, for example, that a Besson BBb comp or a Conn souzy may have a .730 or .734 bore with their short leadpipes, and why Cerveny or a St. Pete with their long leadpipes may have as much as an .827 bore.

One thing I did when determining if my Besson bell would fit on my 186 was to measure the outside diameter of the Miraphone leadpipe at the point downstream from the receiver where it would be the same length as the Besson leadpipe. The old Miraphone leadpipe was actually slightly smaller in outside diameter at that point, meaning that the traditional Miraphone .770 valves are not that large at all for the size of the tuba, and makes it even more surprising that the same valve block is used on all four of the better known Miraphone tubas: BBb 186, CC 186, BBb 187 and CC 188.

The same situation occurs in comparing cornets to trumpets. The typical cornet leadpipe wraps around longer and is more conical before entering the block than a trumpet, so that, at least for vintage King instruments, the same .458 valve block was stamped "medium" for cornets, and "S2" or "medium large" for trumpets (I had one of each until Dad's trumpet was stolen. Please see my signature.)

bloke wrote:Are they now making 186 tubas that big?

.770 has been the bore of 186 valves since the beginning. When a few years ago Miraphone increased the bell from the 16 1/2 inch stovepipe to the broader flare 17 3/4, the leadpipe went up in diameter and the valve bore actually increased to a nominal .772. And my leadpipe is smaller at the receiver with a little more taper than most, being on the recording bell variant. So the current leadpipe is probably the same size or larger at the same point however many inches from the receiver I measured.

I believe 184's and 185's were/are smaller bore.

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