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Old 14-04-2019, 14:42   #1
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Straight Thread Thru Hull to NPT Valve

I know itís not optimal. In fact if it werenít for the fact that I know itís done, I wouldnít have believed that it is in fact done, seems pretty stupid
Iíve never done it, but am now faced with having to put an NPT three way valve onto a straight threaded thru hull. Iíve not yet done it, and my question is how do you keep it from leaking? What seals the threads? With NPT to NPT of course itís an interference fit, thread sealing tape, pipe dope etc is largely more to make you feel good than actually provide a seal, but with straight threads going into NPT I assume there will be no interference, so that the thread sealing will be via pipe dope / Teflon tape etc.
So, what is the preferred method? To make matters worse, Iím going to have to accomplish this in water, so being able to firmly hold the thru hull with the Christmas tree wrench isnít going to happen, Iím going to have to hold it from inside of the boat with vise grips, so huge excessive force with a BIG pipe wrench isnít an option, wouldnít be anyway as there is no room anyway.

What Iím asking is what is best used to provide a seal? Pipe dope, Loctite, Teflon tape?
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Old 14-04-2019, 14:47   #2
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Re: Straight Thread Thru Hull to NPT Valve

Just use an adapter:
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Old 14-04-2019, 14:58   #3
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Re: Straight Thread Thru Hull to NPT Valve

NPT to straight thread will form an interference fit. Yes you can use teflon tape. Loctite makes some REALLY expensive goop which will seal a straight thread to straight thread (BSP) joint which is hand tight, and keeps the fittings from turning. It will be optimal for your situation where you worry about the thru hull turning when you are tightening the joint.
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Old 14-04-2019, 15:17   #4
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Re: Straight Thread Thru Hull to NPT Valve

Don't know if they have it in USA but Heldite & threadtape works good. Put Heldite under & over tape. Or you can use Heldite & plumbers hemp. Its sticky horrible stuff tho. Loctite 518 or 515 works good too or same brand hydraulic pipe sealant. I'm sure Permatex would make something in the line of pipe sealant as well.
Have used all the above methods with success. Probably the best for damaged/mismatched threads is hemp & Heldite but it's also the biggest PITA
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Old 14-04-2019, 15:31   #5
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Re: Straight Thread Thru Hull to NPT Valve

Forgot to say you can build up yr own taper with the teflon tape or hemp too. Agree with previous poster, you can easily get a good seal just hand tight with a bit of practice. Obviously in yr case you don't want to be practicing lol
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Old 14-04-2019, 15:50   #6
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Re: Straight Thread Thru Hull to NPT Valve

i would be more worried about getting enough threads to bite in order give a strong mechanical fit. I think Mainesail mentioned something in his howtomarine.com site.

Be careful.
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Old 14-04-2019, 16:57   #7
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Straight Thread Thru Hull to NPT Valve

Quote:
Originally Posted by ronstory View Post
i would be more worried about getting enough threads to bite in order give a strong mechanical fit. I think Mainesail mentioned something in his howtomarine.com site.



Be careful.


Long story short I am now sitting at anchor with a 90 elbow screwed on top of the thru hull and one heads hose connected to it, while I wait for the three way valve to be flown out here.
It seemed to easily screw way down by just simple finger tight, then suddenly stop, like the threads had bottomed out, and decent torque with a pipe wrench didnít move it at all, I feel sure there was full engagement and the threads did indeed bottom out.
If so then I should easily get it on enough to have mechanical strength.

Now what I am going to have to do is hold the thru hull with vise grips, cause of course I canít let it rotate at all and shear the 5200 bond between it and the hull.
I think Iím going to have to use plenty of Teflon tape, first dry fit and count revolutions as alignment has to be made and I donít want to have to back off any, after the dry fit, tape and go for it.
Of course there is very little pressure, but I donít want a leak.
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Old 14-04-2019, 16:59   #8
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Re: Straight Thread Thru Hull to NPT Valve

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Originally Posted by s/v Jedi View Post
Just use an adapter:


I donít understand, got a link?
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Old 14-04-2019, 17:05   #9
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Re: Straight Thread Thru Hull to NPT Valve

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Originally Posted by a64pilot View Post
Long story short I am now sitting at anchor with a 90 elbow screwed on top of the thru hull and one heads hose connected to it, while I wait for the three way valve to be flown out here.
It seemed to easily screw way down by just simple finger tight, then suddenly stop, like the threads had bottomed out, and decent torque with a pipe wrench didnít move it at all, I feel sure there was full engagement and the threads did indeed bottom out.
If so then I should easily get it on enough to have mechanical strength.

Now what I am going to have to do is hold the thru hull with vise grips, cause of course I canít let it rotate at all and shear the 5200 bond between it and the hull.
I think Iím going to have to use plenty of Teflon tape, first dry fit and count revolutions as alignment has to be made and I donít want to have to back off any, after the dry fit, tape and go for it.
Of course there is very little pressure, but I donít want a leak.
Yep thats what i was trying to suggest about building the taper to stop it bottoming out. Your plan is good except I'd still go for the sealer & the tape. I've done heaps of threaded & welded pressure pipework & the puny pressures u r dealing with should be no trouble
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Old 14-04-2019, 17:26   #10
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Re: Straight Thread Thru Hull to NPT Valve

Groco makes a thru hull fitting that has tapered and straight threads for this reason..... call them.

Greg
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Old 14-04-2019, 17:43   #11
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Re: Straight Thread Thru Hull to NPT Valve

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Originally Posted by a64pilot View Post
I donít understand, got a link?
What Jedi is suggesting is a flanged adapter made by Groco. It is the only right way to do what you want. A lot of people get away with just screwing the tapered threads onto the straight threads but it is a weak connection.
https://www.groco.net/ibvf-1500
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Old 14-04-2019, 18:14   #12
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Re: Straight Thread Thru Hull to NPT Valve

Got it, found it myself but thanks for the link.
I sent an email to the guy ordering parts and asked if he could get me one.
Fingers crossed
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Old 14-04-2019, 20:06   #13
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Re: Straight Thread Thru Hull to NPT Valve

A64--

Typically, if you can get 3-4 full turns, you are fine for pressure force from the pipe. If somethings heavy whacks it from the side in a bad spot, no guarantees.

I would just use put a layer of Teflon tape with with pipe dope on top of it. The dope helps mash and squeeze out all the gaps. Learned that from a plumber friend, he said that once he started doing both... no more leaks during assembly.

Also, wrap the tape clockwise (from the top/end) so tape get stretched into the threads as you tighten.

Can you post a pic of what you are battling? One pic, 1K words, etc. ;^)
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Old 14-04-2019, 20:12   #14
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Re: Straight Thread Thru Hull to NPT Valve

If found the link in Mainesail's site for the thread stuff. It has Jedi's adapter and farther down the page a section on mismatched threads.

https://marinehowto.com/seacock-thru-hull-primer/
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Old 14-04-2019, 20:39   #15
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Re: Straight Thread Thru Hull to NPT Valve

Quote:
Originally Posted by a64pilot View Post
Long story short I am now sitting at anchor with a 90 elbow screwed on top of the thru hull and one heads hose connected to it, while I wait for the three way valve to be flown out here.
It seemed to easily screw way down by just simple finger tight, then suddenly stop, like the threads had bottomed out, and decent torque with a pipe wrench didnít move it at all, I feel sure there was full engagement and the threads did indeed bottom out.
If so then I should easily get it on enough to have mechanical strength.

Now what I am going to have to do is hold the thru hull with vise grips, cause of course I canít let it rotate at all and shear the 5200 bond between it and the hull.
I think Iím going to have to use plenty of Teflon tape, first dry fit and count revolutions as alignment has to be made and I donít want to have to back off any, after the dry fit, tape and go for it.
Of course there is very little pressure, but I donít want a leak.
Hi, perhaps a little left field but would it be possible to retap the female taper thread to match the male thread of the skin fitting? The thread count should be the same, just one is tapered one is not. A plug tap should start easily and only cut a little from the taper section as it cuts its way in. Just a thought!
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