Shawn I'll tell you what I have done with many seacock installs and replacements. I have gone to using a fiberglass backing pad instead of the wood. Wood is easy as you almost always have something around that can be put to use, but it can swell or degrade and cause issues. The fiberglass is solid and won't change but is just a bit more costly. Up side is it bonds great to the hull if you want to do that.
You can make your own backing pad by laying up a bunch of mat with resin, but I don't do that anymore and have sourced the solid glass pad from McMaster Carr. They have a variety of sizes and thickness glass planks and you can go as thick and big as you are willing to pay for. I order a good sized piece and used it for several such projects.
Basically you want to end up with a pad thick enough to actually assist in strengthening the area of the thru hull and proved something for the flange fixing bolts to screw into. The diagram from groco in the earlier post is what you are going for but how wide or long you make the backing pad beyond the fitting is your choice, but don't be too skimpy. I then shape the hull side of the backing pad to fit the hull contour if needed, leaving the flange side perfectly flat paying attention to keeping the top of the pad parallel to the plane of the mushroom head on the outside of the hull. Then prep the hull and pad mating surfaces and epoxy them together. I usually make a nice filet around the joint and of course have rounded off the exposed corners and edges of the pad to make it look nicer.
After this cures you hole saw thru the backing pad using the original hole as a guide. Test fit the thru hull piece for a good fit to make sure it will snug down and has enough threads into the seacock part. When satisfied with the dry fit, break out the caulk of your choice and put it together making sure you end up with the handle in the right spot and direction, etc. and tighten it down good.
Now it is time to do the flange bolts. I no longer put the flange bolts thru the hull but use the method shown on the right side of the diagram, just drilling into the pad and tapping the appropriate thread, holds just fine. (Another reason to use fiberglass over wood pad). Just drill right thru the flange mount holes into the glass. Make sure you mark your bit with something so you don't go blasting thru the hull and take your time. The glass will tap easily with a decent tap. Then it is just installing the holding bolts AND a bonding wire using the bonding screw which is on any good seacock and you are done.
I thought I had some pics of the last ones I did on my boat but don't know where they went. If you really need one, I will be back to the boat soon and can take a couple and post them.
