screw thread issue

[benòKoroit]

I have a Yamaha YEB321s Eb bass with a strange issue with the screw threads on the top of the 2nd valve casing. After a good wash (no specific cleaning to these screw threads) I noticed - when screwing the second valve back in - that it got to a point where the screw was extremely tight and difficult to turn, but after a few rotations began to smoothly twist the rest of the way to the bottom. The issue is with the casing itself, not the cap, as I have tried all the caps with it. I have also noticed that spit is leaking mostly through this valve and out the bottom now, which it hardly used to do before the cleaning. This is probably just a case of someone (probably me) damaging the threads after trying a bit too violently to unscrew a stuck valve cap, but I thought I would just put this out there to see if anyone had any other nuanced suggestions as too what could have caused this.

[bloke]

The Yamaha casing design is tenuous. The pistons fit all the way to the very top, the male threads are - thus - very thinwalled, as (again) there is no relief between the threads and the casing fit.

Any of the regular methods for coaxing male casing threads back to working well again must be done with extreme care - to avoid coning in the male threads on the casings (which would define that the pistons could neither be inserted nor removed.

I have made it no secret that I am not a big enthusiast of Yamaha, and not particularly fond of repairing instruments that they make, as it often takes longer - than other makes - to get them back in the case and to label them “repaired“.