How to unscrew a stuck tuba stopcock sunhat?
Answers: I would not use a torch or a lighter but to some extent a heat gun. Heat it benevolently as well, Too hot and you will wreak a discoloration. Also see if you can get some pointed oil contained by there.
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Wow, that's hideous. The only item I can suggest as a last resort is a torch. I know that sounds loud but it will shrink the brass allowing the parts to free.
Maybe just try a butane lighter and steam the connection for a few moments, and afterwards break it loose. Metal especially brass and copper will shrink making most tight fittings give style.
The guy below me suggested steam, and a good Idea if you hold a worthless type horn. Steam will oxidize the valvles in hurry.
Before the torch, try using hot steam from a (heated)humidifier or from piping from a hot kettle for several minutes.
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