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I have noticed, that, generally, as tuba size increases, so will the bore size, which makes sense, however I have seen 3/4 piston horns with a bore as large as .750 (Wessex Stumpy) and 5/4 tubas with bores as small as .689 (Kanstul 900), so bore size is not necessarily correlated with the size of the bugle.
I have also noticed , in piston tubas, a few commonly used bore sizes that appear on a variety of piston horns of various sizes and keys:
~.690: King 2341 (BBb 4/4), Eastman/Getzen G50/Conn 52J 4/4 CC, Besson et.al. Comp. Eb 4/4 Tubas, Kanstul 3/4, 4/4 and 5/4 horns in all keys, Yamaha 3/4 tubas in all keys, etc., King/Olds/Jupiter Sousa
~.730: Yamaha 321 5/4 BBb, Yamaha Sousa, Besson Comp. 5/4 BBb, Jupiter 4V sousa, Jupiter 4/4 BBb and CC tubas, Conn 20K sousa , BMB,3/4 and 4/4 tubas in all keys
~.750: BMB 5/4 and 6/4 BBb and CC, Kanstul Grand tubas in all keys, Wessex BBb 6/4, Meinl Weston Fanfer 5/4 BBb, Baer and Thor 5/4 CC
~.760+: Yamaha/Meinl Weston/Miraphone 5/4 F Tubas, Yamaha/Meinl Weston/Miraphone 5/4 BBb and CC tubas, Conn/Holton/York BATs
Obviously, the right valveset bore for a horn depends on the specific taper and characteristics of the bugle, not just the quarter-system size definition (which is subjective) and bell size, However, there seem to be a wide range of horns, in every key and size, represented in each category above.
This leads me to my hypothetical question:
If you could only use ONE size bore valveset to attach to a 3/4, 4/4, and 5/4 tuba bugle (Bass and/or Contrabass) what bore (out of the general sizes above) would you use and why? Or would you use a bore size not listed?
Why am I asking? I’m trying to see if there is a general consensus on a ‘universal’ bore size, or which bore some think is the most ‘versatile’ in that it can be used on a variety of different horns.
Again, I realize that the right bore for a horn depends on a lot of factors, and it is seldom as simple as what I’ve laid out here, I just want to hear peoples’ thoughts on what they believe the best Bore/Bugle combination is.

I don't think bore size is nearly important as bugle taper (although it is still an important factor).

Martin 6/4 BBb tubas sport a .712 bore, and play great; York 4/4 BBb (side action) tubas have a .750 bore, and they play great, too (the 4/4 BBb top action has a .656 bore, and they play well, too).

I seem to recall on this forum a 6/4 Holton paired with a King .687 valveset that is reported to play very well, also.

I see, generally speaking, the King .687 bore being used much more frequently than others, but I'm not sure whether that has more to do with availability than anything else.

I've always been a "fan" of the old Conn .734 four valve side-action set, but the general unavailability of them means I've never been able to put one to use.

I suspect that bore selection (back in the day) had more to do with proprietary reasons, and to availability to a lesser extent, although I'm sure that there certainly was some effort made to pair the correct bore to a specific horn.

(As a side note, oftentimes bore size is dictated by the tubing size where the horn meets the large leg of the MTS - for example, it would be quite difficult to paste a .750 set on a Conn 5J, because the horn is .687 where it would meet a tuning slide at .807).

KiltieTuba wrote:The taper is more important than the bore.

I agree, and this is mentioned in the original post, however, my question was, what single valveset (if any) do you think would be the optimal choice to attach to on all of the following if you could only use one: a common (think Yamaha 103) 3/4 bugle, a common 4/4 bugle (think king 2341), or a common 5/4 bugle (think kanstul 900)
.68X could be a valid answer because kanstul puts a .689 valveset on most of their tubas, but a lager bore could also work, and so on. I'm just curious of people's opinions on this matter, and I know that bore size is not the most important factor when determining how a horn plays.

Thanks for the responses so far.
Perhaps I was not clear enough in my original post. To clarify:
I am not looking for a single answer which is correct or incorrect, as I know one single answer doesn't exist and the playing characteristics of a horn depend on an infinite number of factors.
I instead, I'm looking for opinions and preferences of bore size, nothing more.

KiltieTuba wrote:The taper is more important than the bore.

Yes. An aspect of this observation: length of lead pipe. For example, the traditional King 1241/2341 bore is .687. But a King tuba has a short straight-in leadpipe. The traditional bore of a Miraphone BBb 186 & 187 is .770. But it has a long leadpipe that curves around the bugle before going into the block: more room for the taper to continue expanding. I dare say due to the taper, I surmise that the measured i.d. at any particular point along the way, the same distance from the receiver, will be similar.

.730 was the traditional bore for Besson BBb tubas and .734 was the traditional bore for Conn Souzys, incl. 14K, 38K and similar.

Most European and Asian manufacturers have gone with the .728, which is 18.5 mm.

bloke wrote:.709" almost never...184...anything else...??

.709 would be 18mm. The closest I've seen to it in inches would be .718 (23/32, also SAE) which was used on Martin tubas, and I suspect on the York model 692 in CC.