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Compass Marine How To | all galleries >> Welcome To >> Replacing Thru-Hulls and Seacocks > A Good Idea !!
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A Good Idea !!

While both this photo, and the previous photo, were taken on my workbench the plywood is representative of the hull of your boat and the bronze fittings of two very typical installation methods used today. In this photo one can easily see the major differences in how both of these installations are affixed to the hull.

This photo is representative of a proper seacock installation with a through-bolted flange and the previous photo of a not-so-good installation method with no flange and mismatched threads.

Nikon D200
1/60s f/7.1 at 70.0mm iso800 full exif

other sizes: small medium large auto
Guest 21-May-2016 21:16
Thank you, you may have saved my life
Roland Lloyd 07-Aug-2012 13:01

1.If I use a wood backing plate, what type of wod is the best? Marine plywood or a hardwood. What species of wood. What thickness? (note:the old gate valves had no backing plates)

2. Also, what Sealant is preferred? I used 4200 on a same setup 1.5 " Groco thru hull hull a few years ago.

3. What bolts or screws should I to the backing plate? I did not bolt in previous installations, just sealant oand good trque. I do not want to drill thru the hull.

My boat is a 1972 Tartan T34C with thick glass.

This new thru hull I am installing is a 3/4" head water intake. The new Groco valve is 3/4 inch(the type valve like in your instructions). The existing thru hull is a flush- mount fitting, tapered 45 degrees, 2,5 inch outer diameter, straight thread. I had to get an Buck Algonquin fitting with the 2.5" outer diameter to match the Groco because Groco only makes a 2" outer diameter 3/4" tapered fiting but the Algoquin will work fine. (This fitting is hard to find by the way. Middle River Marine in Maryland has this fitting).
Anymore special concerns for the tappered thru-hull installation. Should I use 5200 or 4200 sealant on the outside?
Your instructions are geat!

Joe 14-May-2010 18:46
Where do you get bronze bolts?Can you use brass bolts?