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Some folks also use vacuum cleaner belts for these if you don't have a ready source of O-rings. There's also a stock Yamaha part for these, as well as other second sources, such as Ferree's Tools.

There's some debate as to cork-vs.-neoprene-vs. silicone, but it all works the same way.

The horn I recently acquired has cork on the return side of the rotors. S'alright. I haven't tried it on the far side yet to see if it might be a little thumpy over there. I'm thinking it probably would be.

I'm still using Ashbory bass strings (silicon rubber) on the other horns.