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Old 20-04-2007, 18:29   #1
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Seacock NPS thread: NPT fitting

Hello

Wondering whether it is acceptable practice to screw an NPT-male fitting into an NPS-female thread, when dealing with through-hulls.

I know its never ideal, but wonder whether it can work in practice.

I notice that threaded through-hull's come in NPS, and many seacocks have NPS thread at the bottom, and NPT at the top. However, I have a seacock (Marelon) with NPS bottom and top, and wonder (using pipe sealant and/or teflon tape) what the downsides may be of screwing in an NPT fitting.

In brass fittings with fuel systems, I have been advised by a mechanic that, provided you work at low pressures, it's ok to thread a male NPT into a female NPS. The NPT tapered thread will 'lock' once its diameter exceeds the NPS straight thread's diameter.

However, there may be reasons (unknown to me) why this is not done with large diameter pipes like seacocks.

Any advice or references to archives/pertinent web pages appreciated.

Martin
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Old 20-04-2007, 21:59   #2
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Quote:
Originally Posted by sildene
I notice that threaded through-hull's come in NPS, and many seacocks have NPS thread at the bottom, and NPT at the top. However, I have a seacock (Marelon) with NPS bottom and top, and wonder (using pipe sealant and/or teflon tape) what the downsides may be of screwing in an NPT fitting.
Are you sure????

The flanged seacocks have a straight thread at the flanged end. Because flanged seacocks are sealed by sealer between the inside and outside of the hull with the thruhull. But, the seacock should have a tapered thread on the opposite end.

The only way a straight female thread could seal is with a face seal. Even if you screw in a tapered thread it would probably run out of thread before it sealed.

If the seacock (top) has a large face area you could use a straight nipple with a jam-nut and sealer (Rectoseal-fast dry) and that should do the trick. Then you can screw a tapered female on top of the nipple.

BUT, personally I'd go get me the right seacock, one with a tapered female top end. The fewer fittings the less chance of a leak.............._/)
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Old 20-04-2007, 22:58   #3
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You are right, the Marelon ball valves use straight threads on both ends. Ideally, you should use straight threaded fittings in them. They sell straight threaded fittings for them in Marelon and they are sealed with Teflon tape or paste just like NPT fittings. In practice, it is possible to thread tapered fittings on to them and to get a seal, but I strongly recommend against it because you may damage the threads and the connection will be weaker than if you use NPS. I believe that the Marelon fittings use straight threads because they are stronger and make up a bit for the relative weakness of the plastic. Is there some reason you need to use an NPT fitting on them?

-- Tom.
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Old 21-04-2007, 02:38   #4
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Not a good idea! The tapered thread will lock up when the diameter exceeds the straight, but locking up and sealing are two different things. Only one or two thread will make decent contact. Using pipe tape or other sealant might make it seem to work for a while but why would you take a chance with this on a through hull, which I assume, is below the water line?

Find a nipple/adaptor with tapered thread on one end and straight on the other.

George
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Old 21-04-2007, 05:27   #5
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NPT - NPS adapter used in a marine fluid handling system to prevent thread mismatch between a thru-hull fitting and in-line valve. The adapter includes a head with a male NPT thread and a bore with a female NPS thread.Marine valve adapter - Patent 20060225799

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Old 21-04-2007, 07:39   #6
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Just to explain more - I'm attempting to fit a T-piece at the top end of a water-intake seacock (to prod obstructions from the through-hull while underway). I've so far been unable to locate a 1" T-piece with NPS threads, or an NPS-to-NPT nipple (nipple would be most versatile solution), therefore the question about coupling NPT direct to NPS.

Martin
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Old 21-04-2007, 11:56   #7
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As a Machinist and Hydraulic Specialist, my suggestion now is to get a straight pipe nipple and glue it in place with 3M 4200 (quick dry). Straight threads will not seal 100% on their own without a sealed jam-nut.

Just make sure that the nipple doesn't screw down into the seal or ball of the valve. If its a straight female thread there should be a shoulder inside for the thread to stop. If not, your going to have to use the jam-nut as I mentioned eailer.

As for the "T" fitting, a tapered female will seal on a straight male schedule 20-40 or plastic.

Note: Don't over tighten pipe fittings!! A common error amoung most. Teflon® tape works great for small fittings but for anything over 1" I recommend using Rectoseal #5. I use the stuff on almost everything, including Biodiesel, gasoline, hydraulic and air fittings..........................._/)
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Old 21-04-2007, 15:11   #8
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Have you taked to Forespar?

Marelon is a Forespar product and they have user guides and such on the web. their catalog is at : http://www.forespar.com/onlineCatalo...lonCatalog.pdf. They also have an installation guide that talks about threads.

I like Marelon fittings, but unless you have bought one already and can't return it, wouldn't the easiest solution be to buy a bronze body valve?

Just by the way, all standard threads, straight or tapered will leak if dry fitted. Straight threads can be can be sealed with teflon tape up to at least 1.5" and if done properly bottoming out is not a problem at least at the pressures involved in through hulls for surface vessels. I've done this and it works well in practice. It is possible to make water tight seals between straight and tapered threads and I've seen lots of installations where a tapered ball valve is threaded directly onto a straight through hull. It is, however, very bad practice.

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Old 21-04-2007, 18:24   #9
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Putting a plastic fitting over a tapered threaded metal pipe, can also lead to the tapers causing too much uneven force on the plastic, expanding then eventually splitting it. If you don't overtighten it, if the plastic is robust enough, etc., there may be no problem--but sticking to the correct thread on both sides eliminates the chance of a problem down the line.
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Old 22-04-2007, 21:11   #10
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Thanks for replies

I much appreciate the input - every single reply had something to learn from. Clearly lots of experience in this forum.

I've clearly being cautioned against coupling a tapered NPT directly into a straight NPS, so I'll have to go for one of the other suggested solutions. I'll also call Forespar and ask what they propose - if they have a solution not yet mentioned, I'll post it here.

Regards
Martin
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