I’ve had a lot of emails and questions regarding the mismatching of different thread types such as NPS (NPSM) and NPT.
Someone actually challenged the idea that mismatching threads was somehow OK because “So many builders and people do it.” 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 the best idea. Mismatching threads is also not ABYC or UL compliant.
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 and as such they are not intended to seal or take pressure like NPT threads.
Manufacturers such as Groco warn against mismatching threads as does the ABYC and UL.
“Threads used in seacock installations shall be compatible (eg. NPT to NPT, NPS to NPS).”
Mismatching threads is done by lots of people but as you can see you don’t get a lot of purchase compared to using compatible 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.
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…