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Thanks to Dan Schultz for telling me about a certain Silicone O-Ring stock for rotary valve horns from McMaster-Carr. Red 1/4'' High temperature Part# 96505K25 will fit as bumpers for 186/187 tubas and probably more, if not all others. It will last forever and feels like pressing on a pillow they are so soft and quick. Kudos Dan for finding this stuff...maybe it's old news, but they work and I recommend them.

https://www.mcmaster.com/silicone-o-ring-cord-stock/

I am using 1/4" Soft Buna-N cord stock, also from McMaster-Carr. I bought a 3-ft quantity - I am good for a while. I also received a few other types of cord stock with a tuba I bought a while back.

Perhaps we could send each other a few inches - as an educational exchange...

That size is plenty large enough, but - just for sake of appearance, and nothing else - I manage to shoehorn 5/8” diameter into those and similar applications, so – when I trim it for alignment, it has that “manufactured“ shape to the bumpers, due to them fanning out past the insertion shapes.
Again, it makes no real difference.

bloke wrote: ↑Fri Dec 17, 2021 8:32 am I manage to shoehorn 5/8” diameter into those and similar applications

That ought to be plenty big!

Dan Schultz wrote: ↑Sun Dec 19, 2021 3:56 pm

bloke wrote: ↑Fri Dec 17, 2021 8:32 am I manage to shoehorn 5/8” diameter into those and similar applications

That ought to be plenty big!

Once muscled into place and trimmed, it offers this o.e.m.-ish appearance:

bloke "It's only a 'looks' thing..."

One order is a lifetime supply for most of us!

Dan Schultz wrote: ↑Sun Dec 19, 2021 3:56 pm

bloke wrote: ↑Fri Dec 17, 2021 8:32 am I manage to shoehorn 5/8” diameter into those and similar applications

That ought to be plenty big!

You can say that again!...

LOL...Of course, I meant to type 5/16ths.