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I have a new (to me) Weltklang B and S Symphonie F tuba, and since so many of you play these, I though someone might have the answer to my question!

A few of the bumpers need to be replaced. On prior rotary tubas I've owned, which were much larger, I've used 1/4 inch Buna O Ring cord, and like the way they feel as bumpers. However, looking at these valves, 1/4 inch seems too big. Do you all just squeeze the 1/4 inch down and trim, or do you usually use 3/16?

Since I know the valves are different for different Symphonies, I have a couple of pictures below. Thanks!

Hey daktx2,
congrats on the Symphonie F!

Most Symphonie use about 5.80mm so a smidge less than 6mm. Most custom O-ring or cord shops will have this size. Or just use 6mm and cut down.

Cheers from Berlin,
TheBerlinerTuba

Without attempting the impossible, I roll in something a little bigger with a dull household size slot screwdriver (so as to avoid cutting into the rubber), so that it gets compressed in the C-shape and stays. After that (and trimming the top and bottom), I cut the front end edge off nice and straight at the right place - ending up with some bumpers that were originally round, but after finishing pushing them into their places and trimming them up, they completely resemble those that you've see on Bach trombones and Holton horns.
Whether or not this is the best way to do it, they sure look good.

I see people using Exacto knives, but even those get dull pretty quickly, and I have an easier time just using cheap single edge razor blades, making two or three cuts on each end of one blade, and then tossing them into the trash and grabbing a new blade.

If I want to do some really nice work, I take the cork plates off the instrument, wrap some shirt/cereal-box cardboard around the flat half-moon, and insert that part of the cork plate into the vise.

... if it's a school instrument - whereby others have already put a bunch of careless scuff marks and gouges in the rotor casing top, I'll go ahead and install them without removing the cork plates from the instrument.

my OWN B&S truly lovely instrument...??
A couple of the rubber bumpers have been shimmed out with emergency 1/4 inch long vertically inserted pieces of paper clip, that were jammed behind the bumpers. I keep thinking about replacing all the bumpers on this instrument was nice new ones, but who's going to pay me to do that?
(...and some people probably think I'm joking...)