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Discussion Starter #1
I have purchased new thru hulls and seacocks for my Catalina 30.
The new thru Hulls are smaller (both in diameter and width to the originals).
Thought I had it worked out with distributor but miscommunication resulted in this situation.
Diameter difference old 2-3/8" new 1-1/2"
Width difference old 3/8" new 1/4"

Any suggestions on best way to fill in area around and secure new thru hull.
 

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That is a huge difference in diameter and an even bigger difference in area of the opening. Assuming there was a reason they used the large one in the first place (what are these for? Head plumbing?) I'd be extremely hesitant to make this switch.
 

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Since those are tapered flush mount through hulls, the only real fix is to laminate up layers of glass till it's flush, fair and than redrill and countersink for the new through hull. I had a similar problem only the new through hull was not a flush mount and had a large enough diameter that it overlapped the counter sunk part enough to just fill the void with caulking.

If the old through hull is bronze could you reuse it with the new seacock??
 

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Discussion Starter #8
Since those are tapered flush mount through hulls, the only real fix is to laminate up layers of glass till it's flush, fair and than redrill and countersink for the new through hull. I had a similar problem only the new through hull was not a flush mount and had a large enough diameter that it overlapped the counter sunk part enough to just fill the void with caulking.

If the old through hull is bronze could you reuse it with the new seacock??
The thru hulls are all composite and are probably original. The seacocks were a combination of bronze and plastic.
Seemed like a good idea to change all at the same time and I was trying to stay away from metal below the water line,

Even if the originals could be used the thread size is different. Thanks for your feedback.
 

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Photos of both thru hulls
Hi, if you were to have the old fitting machined to act as a washer for the new fitting through it, effectively acting as a washer to reduce the old hole to the correct size for the new fitting then when installed with an appropriate sealant it should work exactly as planned. You could probably achieve this by cutting the head off the old and counterboring for the new, but a lathe would do a better job. The thread dimensions appear to be similar, although not the same so precludes using the old fitting with new seacock but it should fit perfectly, if done right.
 

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Presuming that your backing plate is permanent (i.e. not wood) like G10, you just seat the backing plate with the new outer threaded fitting tight into the inner seacock fitting... all with epoxy mush. Once cured for a hew hours, grind off the proud outside part. i.e. grind off the protruding thru hull flange and the hardened epoxy that oozed out around it. All smooth and strong as the hull itself or stronger.
This type/model of seacock can be rebuilt from the inside, BTW.
We replaced all of our 80's Marlon originals like this in the 90's, when these new-generation Forespar products had replaced their earlier Marlon ones.

No worries about electrolysis and waaaaay stronger than the ABYC minimum spec for seacock strength.

The base mounting mounting system is clearly visible in this blog. Note that the epoxy was smoothed around the backing plate. Strong and water proof. Gotta say... that Forespar winterizing thru-hull fitting is really great, too. Use it every season.
Coupler, Shaft Seal, and Antifreeze - Blogs - EY.o Information Exchange
 

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Discussion Starter #13
I think I'm with you except for the hull side. The new skin is much smaller than original. Are you saying that the new thru hull will hang below the hull and to apply epoxy around it. Then when cured grind off excess? Thanks for your imput
 

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I think I'm with you except for the hull side. The new skin is much smaller than original. Are you saying that the new thru hull will hang below the hull and to apply epoxy around it. Then when cured grind off excess? Thanks for your imput
If it is the same (or a tad less) diameter as the hole, it will suck up tight with thickened epoxy all around the thread, against the G10 backing plate, and oozing out around the outside of the head (no matter what style of head). Once hardened all the 'proud' part outside gets ground flush with the rest of the hull. Basically the thru hull part has become (Zen alert...) one with the frp hull.
 

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There is a 2" gap between my West Marine Perko and Groco 1-1/4" Thru-Hull and the Seacock
Who here has a 2" thick hull.
West Marine doesn't sale anything smaller and i can't find anything online
Our hull is 1" solid glass, do i really need a 1" thick backing plate to make it fit?
 

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Thanks roverhi.
Just to make sure i'm hearing you right, what your saying is to cut the brand new Thru-hull in half to make it fit the brand new Seacock? All because my hull isn't over 2" thick like a shrimp boat.
 

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There is a 2" gap between my West Marine Perko and Groco 1-1/4" Thru-Hull and the Seacock
Who here has a 2" thick hull.
West Marine doesn't sale anything smaller and i can't find anything online
Our hull is 1" solid glass, do i really need a 1" thick backing plate to make it fit?
It might be helpful to harmonize our terminology in such a thread, too.
What you describe is a "valve screwed onto a threaded thru hull" and not actually a "seacock". The later sits lower and is a one piece casting incorporating a valve (usually a ball valve these days) and the hose is clamped directly onto the tailpiece. This is very very strong compared to a valve screwed onto the thru hull fitting.
And, a seacock also sits low against the hull inside surface. If your hull's layup is actually one inch thick in the installation area, yes, you already have the equivalent of a 'backing plate'. Of course that is just an opinion flung out from the internet - worth 'bout what you paid!
:)
 

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Thanks roverhi.
Just to make sure i'm hearing you right, what your saying is to cut the brand new Thru-hull in half to make it fit the brand new Seacock? All because my hull isn't over 2" thick like a shrimp boat.
Yes, cut the thru-hull to whatever length works with your hull thickness. You could put an over thick pad under the valve but it wouldn't be needed for strength. If you go the thick pad route would not use plywood. Used plywood when we built our boat and most of them delaminated over time. You could saturate the plywood with epoxy but that's not a guarantee that water still will not play havoc.
 
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