Many boaters have asked me how to install stronger, flanged seacocks without also drilling extra holes in your hull for the bolts that hold the flange. It can be done, as you will see below. Though this is going to be slightly less robust, in strength than through bolting, I would have no qualms about using this method on my own boat. I do admittedly through bolt on my own vessel out of personal preference. Don't get me wrong this is still a robust installation, if done with care & patience, and it is stronger than the hundreds of thousands of vessels out there with noting more than a valve threaded to a thru-hull. This is also a decent option for those with cored hulls where thin external skins are to thin to take the countersink depth of the bolt head.

This article already assumes you're using GPO-3 polyester/fiberglass or G-10 epoxy/fiberglass sheet for the backer blocks, and have already made the circle/donut.

This article also assumes you are utterly opposed to through-bolting your seacocks to the hull.

For tapping I recommend a thickness of about 5/8" for your GPO-3 or G-10. In testing I have exceeded 1000 pounds of direct pull on 1/2" home made polyester fiberglass board. G-10 and GPO-3 are far stronger than my home made fiberglass sheet and the extra thickness yields even more strength.

Unlike the other method where you glass the backing plate to the hull first then drill the hole for the thru-hull with this method you drill the thru-hull hole first. To do this project a drill press is HIGHLY recommended.

Before you drill and tap the backer block you need to tighten the flanged adapter to the backing block using the thru-hull. Because you are missing the thickness of the hull you will need to create a spacer that takes the hulls place.

I used some PVC exterior trim lumber and drilled the same diameter hole as in the backing block. Then all I did was center & tighten the flange and thru-hull together with the backing plate and spacer sandwiched in between.

Please don't cut corners on the quality of your tap and always use the proper sized drill for the tap.

This is an "F" drill for a 5/16" X 18 thread tap. You can also use a 17/64" drill with a 5/16" tap. Fiberglass or epoxy sheet is susceptible to off center tapping so be careful to not cut crooked or wobble your tap side to side as it will make the tapping bigger than you want. Practice in a scrap piece of fiberglass first if your not comfortable with this.

Once tapped through use an air gun to blow the flutes of the tap out. Do this before reversing it out of the fiberglass..

Silicon bronze threaded rod is expensive but silicon bronze carriage bolts are cheap. Simply install two nuts onto the bolt and clamp only one of the nuts in the vice while tightening the other back onto it. This essentially locks the bolt into the vice so you can cut it without it spinning or moving.

As you get good with the Dremel you can make some amazing bolt cuts with threads that work as good as factory. Of course you could use a hack saw too but the Dremel is nice.

With the studs made you simply thread them into the backing block. If you got a little aggressive with the tap it's not the end of the world. Mix up some slightly thickened epoxy and apply with a syringe into the hole, wax the stud and thread it in with the thickened epoxy and let it cure. This will create a nearly perfect thread interface between the stud and backing plate. Even with a good tap I still dip the stud into epoxy, coat the inside of the hole and thread it in.