[will.vermeer67]

I made one just now. I took the instrument (baritone sax) including the neck to our local hardware store and searched for the parts that would fit and work.
I soldered the first metal washer to the bolt head (soldered all around/air tight) otherwise the tool leaks on the inside and it would not work.
The washers were a tiny bit to big on the outside so I grind them down to smaller size.
Tightening the nut by hand. not a wrench. You could damage the sax. I use cork grease on the black sealing rings to help for an air tight fit.
After I put the leak checker in and tightening the nut I placed the neck and when blowing on the neck there was a clear air stream through the flex hinge of the receiver.
So conclusion: neck joint is leaking!
Bolt size: M12x50, sealing ring outer diameter 25mm. bush: cupper hose, it happened to be available: length of cupper bush: 26mm,
washers grinded back to outer diameter 21 mm, maybe 22 would have been enough (experiment yourself!, good luck).