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Compass Marine How To | all galleries >> Welcome To >> Seacock & Thru-Hull Primer/Pre Information > Mismatched Threads !!!!
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Mismatched Threads !!!!

I've had a lot of emails and questions regarding the "mismatching" of different thread types such as NPS (NPSM) and NPT.

Someone challenged the idea that mismatching threads was OK because "so many do it" so last night I made a cut away view to show why using two different thread types is really not such a good idea even if "so many do it".

While many boat owners & builders have screwed NPT threaded valves directly onto NPS threaded through-hull fittings it's clearly not a logical or the best idea. NPSM or NPS, as it's referred to, is the actual sub class of straight threads used in the marine industry. The M means "mechanical" seal they are not intended to seal or take pressure like NPT.

Manufacturers such as Groco warn against doing. Mismatching threads is done by lots of people but as you can see you don't get a lot of purchase compared to using matching threads. All threading machines have slightly different tolerances and some straight thread might screw in more and some may screw in less. The parts in the photos were purchased right off the shelf at my local chandlery.

To make this photo possible I basically used a bronze nipple, or threaded pipe, with standard plumbing threads of NPT (National Pipe Tapered) and a bronze coupling also the industry standard of NPT thread.

I cut the bronze coupling almost in half, for a cut away view, so I could thread the NPT bronze nipple into one side and an NPS through-hull fiting into the other. I then sprayed each with a McLube, to reduce friction but save the picture quality, and threaded both the NPT nipple and the NPS through hull into the cut-a-way bronze coupling by hand and until I had an equal resistance.

The results even surprised me. As you can clearly see the NPT nipple threaded into the NPT coupling a quite a bit further than did the NPS through-hull. One would expect this because NPT and NPT threads match. If you were to take a wrench to both you might get one more turn at best out of the NPS through hull but you may still get two or three full turns out of the NPT nipple.

If you look very closely at the picture you can also see the outer-most threads of the through hull are already NOT fitting tightly against the female threads of the coupling and the inner-most threads are quite tight or virtually bottomed out...!!! The square peg evidently does NOT fit a round hole.

This coupling represents the threads of an in-line valve. Most all available ball valves or gate valves have NPT or tapered threads and most all commercially available through hulls have NPS or straight threads which is a clear & potentially dangerous mismatch.

Sticking a ball valve directly onto a through hull gives you about three or four threads between sinking and floating so I can't, with a good conscience, recommend you do it. For safety's sake I suggest using proper seacocks with flanges.

I would have cut away an actual ball valve but I don't have a machine shop. The coupling represents and has the same exact NPT female threads as an in-line valve of either the gate or ball type...

Nikon D200
1/60s f/4.0 at 46.0mm iso100 full exif

other sizes: small medium large auto
Ben 11-Dec-2016 03:47
While on the subject of mismatching threads, also be very careful of mismatching BSPT/JIS, British standard pipe thread/Japanese industry standard, fittings. The main difference being the number of threads per inch. I.e. 1/4" fittings, National Pipe Standard measures 18 threads per inch, BSPT/JIS has 19 threads per inch. You'll end up with only 1 1/2 to 3 threads engagement before they start damaging the threads.
Bill Calfee 23-Feb-2009 00:15
I am tearing out 13 thru hulls with ball valves on my HR 38. All except on was threaded on about two threads!! A scary thing. I have everything ready to install just as you have suggested, except that I am bolting into a 6 x 6" block apoxied to the inside of the hul. i could not bring myself to drill 39 more holes in my boat.
Wayne 19-Dec-2008 23:06
Excellent job! But one question. I agree the NPS thread is not a sealing thread. So where does the seal take place. Usually in mechanical work an O ring or some type of gasket is used with NPS connections. Pipe dope isn't the correct solution for a NPS seal.
badsanta 20-May-2008 08:44
You are so right about this. I have had some good arguements about this. I was so happy to find the Groco flange. Its about time. I never liked the way everyone did this. This should be a huge poster in every parts store. This is All I will put on my boat. I have finely replaced them all. I cant beleave it, parts places all have the valve and thru hull, But I have to mail order this adapter flange. If you try it you will never go back or feel safe without one. Thankyoy so much for this great service. Boats will be saved.