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I've read a few great posts on this site about proper installation of thru-hulls, seacocks, and ball values, particularly by Maine Sail. Thank you. So it sounds like capping a thru-hull with an NPT cap is a bad idea. Where can an NPS cap be obtained? I have a sound thru-hull in an extremely well protected location (zero possibility of anything whacking it) that is now unemployed after removal of a heat pump. I've also removed the NPT ball valve that was stuck on the thru-hull. I'm not quite ready to yank the thru-hull out of the boat altogether, so can an NPS cap be had for it?
 

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I've read a few great posts on this site about proper installation of thru-hulls, seacocks, and ball values, particularly by Maine Sail. Thank you. So it sounds like capping a thru-hull with an NPT cap is a bad idea. Where can an NPS cap be obtained? I have a sound thru-hull in an extremely well protected location (zero possibility of anything whacking it) that is now unemployed after removal of a heat pump. I've also removed the NPT ball valve that was stuck on the thru-hull. I'm not quite ready to yank the thru-hull out of the boat altogether, so can an NPS cap be had for it?
Capping a thru-hull with an NPT cap is actually better than capping it with NPS. NPS threads are not intended as a pressure seal and are the same in theory as standard nut and bolt threads.. At least with an NPS thru-hull and an NPT cap the NPT cap is tapered and will create some what of a seal, all be it with less thread contact, provided you use teflon tape and pipe dope. NPS & NPS is not something I would suggest bellow the waterline. I also don't like NPT & NPS mixed threads but it's better than NPS to NPS bellow the water..

I would urge you to consider filling the hole using a proper 12:1 bevel ratio or replace what you have with a flanged seacock which you can then plug and the threads will match. The problem with plugged thru-hulls is that they generally never get looked at, because they are plugged, and then galvanic corrosion sets in and they can fail becaue they have not been looked at to know this in years..
 

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I would consider installing a proper valve on the fitting and install a cap or plug on the valve. You would have double protection that way. I too do not like the idea of mixing the two thread standards.
 

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Double protection or 2x the failure points?
I think what he meant was that by leaving the valve on the thru hull you can close it (safety measure one) and by adding an NPT plug into the other end of the valve (safety measure two) you now have an additional fail safe if the ball were to corrode out..

P.S. This threat is nearly five months old!!!
 

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NPSM to NPSM would actually be fine below the waterline, with teflon tape...

The M stands for "mechanical", the type of seal that NPSM threads create. It is not intended for pressurized systems, but unless your thru-hull is under a certain depth, they are actually safer.

Good luck finding an NPS (same as NPSM) cap though!
 
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