Through hull fittings are always straight thread. Seacocks are always
straight thread. The reason is that there is no way to predict how
thick the planking and backing block will work out to be. You have to
cut the through hull so that the maximum amount is in the seacock with
just a little margin to get everything tight. The flange of the
seacock then keeps it from coming unscrewed.

It would be odd to find a ball valve with a straight thread (I
looked). Even if you could, it wouldn't be a good idea because,
without a flange against the hull to secure to the backing block (even
if only with bedding), there isn't much to keep it from coming
unscrewed. Try twisting hoses onto that set up.

The purpose of tapered threads is to lock tight but there is always
some uncertainty just where they will end up along the axis of the
pipe because of manufacturing tolerances, torque, etc. That's why
they can't be used where a flange is going to stop the motion at some
point.

Putting the straight thread into the tapered thread results in only a
couple of the threads fully engaging. If you put enough strain on it,
those two threads will strip. That's the theory. It's fact that the
straight thread into the taper thread can not develop anywhere near
the design strength of the joint. It's practice that, with the
malleability of bronze, some pipe dope, and the fact that the whole
thing is typically connected to flexible hose instead of more pipe
that people might be stepping on or which is subject to thermals
stress, the joint is strong enough for boat drain purposes. It is
second best and you do need to recognize that protecting it from
impact and stress is much more important than with a proper seacock
that has a flange to mate with the backing block.

Putting a seacock in is pretty quick and easy with a helper. I did
the ball valve thing with my two head connections because nothing is
stored in the area and I was working alone in cold weather that would
have made working with bedding compound difficult. If I was replacing
gate valves in an engine room where someone might step on the through
hull or anywhere else that it was subject to strain, or if it was
connected to pipe instead of hose, I would definitely use proper
flanged seacocks and new through hulls. Why new? You have to pull
them anyway to trim to proper lengths and they don't cost all that
much.



--

Roger Long



wrote in message
ups.com...
Woops s/NPT/NPS/. that's obviously a straight thread sticking up.

-CB