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A little background, I picked up an old typical European 4v small F (A. Vogelweith, Strassburg) in good condition for next to nothing. It is very responsive with unexpected good tuning. BUT, the top of the second rotary valve is broken (see pictures below). This effectively leaves me with a very small butt-joint that needs to be fairly strong, and I assume that a simple solder-attempt would not be strong enough.
The only solution I have been able to come up with is to take a suitable-sized brass screw rod and solder it in place inside the two halves. While this will likely make the valve structurally useable, it would mean that I can not use the center hole for the screw that holds the valve-arm in place.

As this must be a fairly common problem, what is the usual solution for a functional repair (besides taking it to a professional tech)?

Any hints are much appreciated!

Take it to a good shop. Have them braze/silver solder/hard solder the nub in place and do any work needed to fit it to the casing. (It is likely none will be needed.) This is a fairly easy and fast fix but needs to be done by someone who is experienced at the task. This is not merely fixing a break, but fixing a break that must also drop in and out of a tube and rotate.

The shop might (If not already threaded) tap the same threads down into the drilled part of the rotor (the same threads which are in the stem) and screw in a piece of threaded rod (which still leaves room for your stop arm screw). This will help them to line up the two pieces a bit better prior to reattaching them.

A friend of mine had an (aluminum ) axial-flow (trombone) valve stem break off quite a few years ago.

I "screwed" the stem back on with a 3mm stainless steel rod coated in (industrial-grade) epoxy.

Even though it was only glued - due to being aluminum (yet reinforced with that threaded rod) it lasted my friend until he "got over" being enamored with the axial flow valve idea. (The same trombone now sports a traditional rotor.)

me...??
I doubt that I can eyeball a busted off rotor stem well enough to not hold it in place (while reattaching it) without securing it with a piece of threaded rod...obviously, it has to be rotated to the correct angle, and - also, if the two surfaces are crusty, the shop will probably either scratch away all the crustiness or grind two two surfaces down smooth.

' tough break !

good luck !!!

Thanks!

Your advice is much appreciated.

I will use this as a learning experience and see how far I can get on my own (before giving up and bringing it to a proper tech).

Yeah.. I wasn't encouraging you to follow my outlined possible repair techniques yourself , but suggesting that as a method that usually works for me - as a somewhat experienced repair monkey.

I believe I would send it to someone who has a particularly good reputation working with/specializing in rotors. If you sent it to me, I would sigh first, look back and forth between you and the instrument, but I would fix it and I'd probably be successful...but there are some people who spend most of their time working on rotors. They tend to specialize in French horn rotors (as would be expected), but still...

I think - with that tuba, you've got one shot, and it would be a good idea to not take it to someone who would blow it or play around with it yourself if you haven't done much silver brazing or putting broken things back together that have to work just so (as opposed to so-so).

Finally, (realize that I'm looking at a two-dimensional picture on a phone screen, but) I think that "no one ever dissolving lime deposits out of that instrument" (ie. no one ever cleaning it) contributed to the stem breaking off. I believe that I'm seeing white deposits on both broken off surfaces.