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Discussion Starter · #1 ·
Looks like Seacocks only come with NPT threads. Is it OK to tap it to convert from NPT to NPS?

I have a couple of thru-hulls that are 1/2". Groco does not provide their little addapter base (NPS to NPT) for 1/2" thru-hulls. They provide a 1/2" seacock with NPT on both ends, but all the 1/2" thru-hulls are NPS.

Thanks ahead of time for the help.
 

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Groco seacocks have NPS threads for the through hull. As does any other seacock I have seen - I sell them. Add a pipe to hose barb fitting on the inboard end and you're done.

The Groco flanged adaptors also have a NPS thread for the through hull with a NPT thread for a ball valve on the inboard end.

Groco does not have a 1/2" seacock - they start at 3/4" as does their flange adaptor.

I wouldn't recommend tapping to change the threads. What are they used for? 1/2" is too small for anything I can think of - especially a raw water intake for an engine. Better to change to 3/4"
 

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Discussion Starter · #3 ·
I have a 1984 35' Bene. The engine water intake, house water intake and the toilet intake are all 1/2". The boat is 30 years old, so maybe it was found not to be sufficient.

The Groco BV series starts at 3/4". Then the base (adapter, IBVF) plates used with the IBV series on starts at 3/4". The ball valve in the BV series has a 1/2" version. So, I decided to stay with the 1/2" valves as that is what was the original installation.

I think next year, I will convert the 1/2" to 3/4" so the IBVF (or BV seacocks) bases can be used.

Thanks for the information. Helped a lot. Wish I would have asked the question back in January.
 

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Seacocks today usually start at 3/4" to meet the minimum ABYC strength standards. 1/2" intakes are also very prone to plugging and causing overheating issues. Best to go one size bigger to 3/4" and install a proper seacock or flanged adapter.

NPS and NPT will mate together, same TPI and pitch, but it is not a "proper" thread match because one is a straight thread and one is a tapered thread. NPS to NPT is not one that is ABYC compliant and some insurance companies are now starting to get picky...

That said there are hundreds of thousands of boats out there floating around with NPS to NPT but it is not the right way to do it. Even if you tapped a valve to NPS you would still have a seacock that is not compliant... Definitely DO NOT run a die over the thru-hull and try and convert it to NPT. The wall thickness is not designed for it...


 

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Discussion Starter · #5 ·
Thanks, makes good sense. Glad I asked. I will convert after haul-out next year.

It is interesting to note these things. The original installations and plumbing sizes are not up to today's standards. I thought I would be up to standards if I kept everything the same. Bad assumption. The only 3/4" fittings are those on the sink drains for the galley and head....the rest are 1/2".

BTW MaineSail (if you read this): I installed a port window using your method except I used SikaFlex 291 with butyl tape. I put about 5 times more SikaFlex than I should have. You should'a seen the mess....will be better for the next one.
 
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