"Roger Long" wrote in message
...
has
anyone ever heard of a NPT valve on a NPS through hull breaking off or
leaking?

--

Roger Long



Or, as another possibility, has anyone rethreaded a NPT to NPS?

Facing replacement of a 2" and wondering...

L8R

Skip

--
Morgan 461 #2
SV Flying Pig
http://tinyurl.com/384p2

"Twenty years from now you will be more disappointed by the things you
didn't do than by the ones you did do. So throw off the bowlines. Sail
away from the safe harbor. Catch the trade winds in your sails. Explore.
Dream. Discover." - Mark Twain

Or, as another possibility, has anyone rethreaded a NPT to NPS?

I asked both a machinist friend and the shipyard and they said it
couldn't be done.

However, looking at the two half inch fittings on the desk in front of
me at this very moment, I'm damned if I can see why running a pipe die
down the first few threads of the through hull wouldn't taper it and
let it engage more than the half depth it does now.

Anybody out there got a pipe die set and want to invest $5.39 (price
of half inch bronze through hull at local store) in edifying the
boating world?

--

Roger Long

"Roger Long" wrote in message
...
Or, as another possibility, has anyone rethreaded a NPT to NPS?

I asked both a machinist friend and the shipyard and they said it
couldn't be done.

However, looking at the two half inch fittings on the desk in front of
me at this very moment, I'm damned if I can see why running a pipe die
down the first few threads of the through hull wouldn't taper it and
let it engage more than the half depth it does now.

Anybody out there got a pipe die set and want to invest $5.39 (price
of half inch bronze through hull at local store) in edifying the
boating world?

The local Lowes and Home Depots will cut threads reasonably cheaply, if you
don't have one. Unless, of course, the threads (not the taper) are
completely different in the two (which hardly seems likely, as people are
putting valves on NPS all the time).

I was thinking more along the lines of the other way - as, surely, given
that it starts that wide, the valve has enough body to support removal of
some material - taking an NPT valve and machining/tapping/whatever the entry
to NPS.

I'd think that taking material away from the TH would weaken it
unnecessarily, yielding the same end result of a small portion of the
full-depth pipe section being available to support any side loading, whereas
taking away the material in the valve shouldn't have any deleterious effect
on the strength. But then, I'm not an engineer, nor even play one on TV so
can't really comment on that.

L8R

Skip

--
Morgan 461 #2
SV Flying Pig
http://tinyurl.com/384p2

"Twenty years from now you will be more disappointed by the things you
didn't do than by the ones you did do. So throw off the bowlines. Sail
away from the safe harbor. Catch the trade winds in your sails. Explore.
Dream. Discover." - Mark Twain

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