I've been looking closely at these ½ inch fittings and measuring them,
comparing the NPT tail piece with the NPS through hull. The threads
are identical to the eye and the taper is only visible with the
parallel sides of measuring tool. Interestingly, the NPT tail piece
goes only as far into the valve as the NPS through hull with hand
force. The NPS fitting hits bottom more solidly however and I'm sure
it would feel differently when honked on.

Relieving the first half diameter of the through hull with a NPT pipe
die would clearly taper it enough to let it go deeper in the fitting.
The threads farther back, left untouched by the die, would then do the
same job as on the unmodified fitting but there would be more
penetration and more threads engaged. This would clearly be a good
thing.

The amount of material that would have to be removed to taper the
first half pipe diameter would be insignificant and there would be
untapered threads buried the same distance in the valve as if the
through hull was not modified.

The shipyard was right, however because they were responding to a
different question. The whole point of the exercise is to avoid the
labor of removing and rebedding the through hulls. Trying to re-thread
them in place usually wouldn't be practical because of space and the
risk of breaking the bedding. Once you have to take them out, you
might as well put in a proper seacock and do the job right.

Tapping the valve out to NPS isn't practical because the straight
threads and only be depended on when they are squeezing something so
as to provide the force to lock the threads.

Since my half inch through hulls, which are replacing plastic fittings
that I'm removing, are not in the boat yet, I may try to get a die run
on the end to see what happens. I want to cut them shorter anyway
later to minimize the leverage on the through hull when the valve is
actuated. If the re-threading looks good, I'll have it repeated. I'd
be using proper seacocks incidentally if these were larger seahulls
but ½ inch doesn't seem to be available.

It's amazing that, after over 30 years in the boat business, there is
still so much to learn about something so basic. I'm pretty sure now
that just putting ball valves on old through hulls is a reasonable
thing to do and this is another case of "don't believe everything you
read". If there were gate valves on there before, the force fit of the
old fittings has probably already somewhat modified the through hull
threads anyway.


--

Roger Long