# Assemble thru-hull & seacock...how?



## TSOJOURNER (Dec 16, 1999)

I have a 1 1/2" ID Conbraco bronze mushroom-head (NPS thread) thru-hull; the length of the threaded shaft is about 3". I also have a Forespar/RC Marine (Marelon) seacock.

1. West Marine catalog (http://www.westmarine.com/webapp/wc...e&storeNum=null&subdeptNum=null&classNum=null) says threads on the seacock are NPS which should jive with the thru-hull. However, the Forespar website, if I'm reading this page right (http://www.forespar.com/onlineCatalog/2007/Marelon/06-flangeMountingSeacocks2007.shtml) suggests threads are NPT. What am I not getting right here? Could I have ended up with a seacock with BSP threads?

2. Dovetailing to above question, when I dry-fitted the thru-hull into the seacock, I can get the first 4 full-turn threads on, but then the fit gets increasingly difficult to turn by hand...to the point where I can't get the thru-hull to move at all. Do I have a thread mis-match here? (I'm thinking about exchanging the Marelon seacock for bronze...only about $12 more).

3. And this is the crux of my post. Our hull (1981 Oday 28) is about 1/2" solid fiberglass. The backer block I intend to install is 1/2" fiberglass which will be epoxied into position. This means I have 1" of "hull thickness" for the thru-hull's tube. But the depth of the seacock receiving end is about 1 1/2". I intend to have the thru-hull bottom out into the seacock and, thus, the seacock flange sitting flush with backer block. Then, I'll sink screws through seacock flange holes into backer block...don't want three more little holes in hull. Can I assume that I will have to cut off about 1/2" off of the threaded end of the thru hull (remember the shaft of this thru hull is about 3") so it can bottom out in the seacock (or get close) as the seacock fully engages to the backer block? Or should I plan to "shim" the backer block?

What is the preferred method for installing these parts? In advance, thanks for your input. Best regards.


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## PBzeer (Nov 11, 2002)

First of all, I wouldn't mate a marelon fitting with a bronze, particularly one that will be below the waterline. You might also check and see if the thru-hull has tapered threads, which is why it will only go on so far.

If you're going to use a flanged seacock and a backing plate and have a 1/2" left over, it would seem easiest to just put another 1/2" piece of fiberglass there to make up the "leftover". Or use an unflanged seacock, especially if there's much curvature at that spot.

Just put one of each type of seacock on new thru-hulls on my boat, and made the choice by how rounded that part of the hull was.

_Back in the water at Deaton's Yacht Service, Oriental, NC_


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## TSOJOURNER (Dec 16, 1999)

*Dryfitted to extra ball valve and guess what?*

I had an extra 1 1/2" Forespar Marelon Ball Valve lying around, so I tried dry-fitting thru-hull on that. Well, threaded all the way up and bottomed out. So, back to the seacock.
The top of this seacock is also supposed to be threaded NPT. So, dry fitted the thru-hull to the top of this seacock and voila! Threaded all the way up.
I am starting to suspect one of two things happened. One, someone bought this and threaded an NPS thru-hull (didn't think they made them) and screwed up (no pun intended) the threads of the seacock, and then returned it (now I've got it)...OR two, Forespar put the wrong "bottom" on this seacock. 
In any event, I think I'll just return it for an Appollo bronze seacock (think I'll take the thru-hull with me and dry fit in the store).


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## sailingdog (Mar 19, 2006)

I would agree that using a Marelon seacock over a bronze through-hull is probably not the best idea. The expansion and contraction of the two materials is probably going to cause problems in the long run... since the bronze probably expands far more than the Marelon, and the Marelon will be in tension around the bronze through-hull. 
*
The bronze through-hull is NPS*, rather than NPT. The Marelon seacock is NPT, and as such will have some problems if used with the NPS-threaded through-hull. Either get a Bronze seacock that is NPS-threaded or get a Marelon through-hull that is NPT-threaded. Please note in the two attached photos that the Conbraco are clearly NPS-threaded, and that the Marelon Seacocks are definitely NPT-threaded.

You would probably want to cut the through hull to length to fit the seacock properly, so you can bolt the seacock down solidly. BTW, you will want to get some plumbing thread sealant. This is the kind I'd recommend: *LINK*


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## badsanta (Oct 16, 2006)

I would use Groco flange adapters. check out groco.net/new-prdcts-home/ibvf-08.htm


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## Maine Sail (Jan 6, 2003)

*If you want a better understanding*

If you want a better understanding read these:

*Seacock & Thru-Hull Primer/Pre Information*

*Replacing Thru-Hulls and Seacocks*

Basically you don't want to mix NPS and NPT which it sounds like you have. Forespar offers Marelon "in-line" valves as NPS but you definitely DON'T want NPS and NPS if the threads are the only thing holding that valve to the thru-hull!

SD is correct in that the mixing of bronze and Marelon is not advised.

The use of "in-line valves" screwed directly to a thru-hull without a proper flange is not the wisest choice!!

NPT vs. NPS









P.S. I like this quote by a fellow forum member:
*"The problem I had was when I went to remove the frozen valve, it turned the thru hull, thus distroying the thru hull seal."*

This should be reason enough to use properly flanged seacocks or as stated above flanged adapters from Groco.


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## TSOJOURNER (Dec 16, 1999)

I have seen bronze through hull fittings that are NPS but are NPT just at the tip so they can fit an NPT ball valve fitting.


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## sailingdog (Mar 19, 2006)

I seriously doubt that... since through-hulls are designed to be cut to the proper length, and you can't cut NPT threaded sections and expect them to fit anything properly.



string11 said:


> I have seen bronze through hull fittings that are NPS but are NPT just at the tip so they can fit an NPT ball valve fitting.


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## AjariBonten (Sep 7, 2007)

Must be commissioning season................. What is this the fourth Thru-Hull thread today??????


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## TSOJOURNER (Dec 16, 1999)

Well I am a machinist, and while I am not a thread expert, I can tell when a straight thread turns into a tapered thread. I don't know if it is the national standard taper and I don't know if the design intent was to make them compatible with a tapered female threaded fitting but these through hulls were definitely tapered at the tip.


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## Maine Sail (Jan 6, 2003)

*While..*



string11 said:


> Well I am a machinist, and while I am not a thread expert, I can tell when a straight thread turns into a tapered thread. I don't know if it is the national standard taper but these through hulls were definitely tapered at the tip.


While there is such a thing, though quite rare and usually special order, it's NOT an NPT thread but rather what's called a "combination thread". A combination thread is a band aid at best. To make a combination thread they start with an NPS thread and then shave the points off the top few threads thus slightly reducing the od of the thru-hull. This still does not allow cutting the thru-hull to a proper length which should be done for safety.

A proper thru-hull has a through bolted or screwed flange flange and is MUCH safer than a ball valve screwed to a thru-hull directly..

*This is the typical wall thickness, after the threads have been cut, of an off the shelf, standard 1" bronze thru-hull!*









*Overall Thickness:*









The thread depth/cut of a 1" thru-hull is roughly 1.45mm. Flanged thru-hulls are MUCH safer just ask me who had a ball valve screwed to a thru-hull crack!!

Oh and do not mix Marelon and bronze. Use bronze/bronze or Marelon/Marelon but not bronze/Marelon.

*NEVER EVER USE HOME DEPOT OR HOME CENTER YELLOW BRASS VALVES!!!*

*This valve was less than 1 year old and the ball was totally GONE!*


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