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I have a Yorkmaster piston valve block and recently the #1 piston valve guide continues to become loose after repeated tightening.

What is the preferred method to fix the screw on the side of the piston yet maintain future serviceability?

Blue Loctite?

On my sousa.. I used these style valve guides so they couldn't untwist

https://topebandsupplyco.com/products/c ... phone-tuba

Assuming these are the usual size valve guides, try nylon #3-48 or M2.5 screws, you can buy 100s on amazon cheap, so if you mess up some it’s not a huge loss, and hopefully end up with quieter valves without wearing out the slot in the casing.