TromboneChat

I was dreading buying a piston valve instrument because I thought all valve caps threaded on like my chinese euph - back thread for 3 minutes with 10 false starts, then eventually it goes on without cross threading. But then all of my DE mouthpieces are easy to thread on, and my 60 year old american euph valve caps all work great. What's the difference between valve caps that thread on the 1st time and ones that dont?

Sloppy threads, of course

Usually too much tolerance between inner and outer threads makes it easily possible to start skew .

Try this. Set the valve cap on, turn in the direction to open with very light pressure until you hear a click, meaning the threads did fall into place, then turn in the right direction to fasten.

Having an adequate lead and pilot to threads is key to not having them cross thread. If you can keep the axis of the two threads square within a couple of degrees it will be fine. The reverse turning above to find the start is a good strategy.

*this also assumes that the threads are co-axial with both the casing and cap... I would not assume either.

Sharp, square threads suck for not getting cross threaded. Even worse if they are loose and have poor tolerancing.

Cheers,
Andy

Worst threads ever were on one of those Giss system Diana's. Well executed German engineering, but those threads were the finest threads I've ever seen and you had to get them started against spring pressure. I dreaded replacing a seal on one of those.

Wish i still had it though.