Like I said, repeating Forespar, Buy the set. Seacock comes with thruhull. Probably comes with printed instructions for the yard manager to follow.
4200 is a polyurethane. I wouldn't use anything but marine polysulfide.
Strangely Forespar recomends any and all brands of marine adhesive including 5200. Will ignore that. but will order a set of the 93 series to see what it is. If you lag in the flange from the top with screws, it may mean you can take the seacock off from the inside. You could replace with a fresh one while under way (and a face full of ocean.)
What you do with the mounting is make sure (if you have the height room inside) that the thru hull fits exactly into the seacock as it comes from the box when it is screwed down to the backing plate - or flattened mound of epoxy. I would do this with any brand of valve, because repair and removal next time becomes easy, and therefor not something likely to be put off.
Scott, we are talking here about a flanged seacock (Forespar 93 series) that has straight pipe threads that engage the thru hull. If you get your A's and B's right, as you say, you will be able to turn that valve right down onto the backing. And have it TIGHT. And it is flanged so just about nothing is going budge it if it gets hit.
5 or 6 turns of matched pipe thread is pretty darn good for pipe and fittings. That is I think one half to three quaters of the threaded male portion (IT IS on the tailpiece that goes into the 1 1/2" Forespar inline valve I have right here. but it only feels right as I'm no engineer.) All the lands and grooves are touching in that small area. That's the advantage of pipe thread - you tlghten with teflon tape - and a little bit more.
The seacock screwed onto a straight thread thruhull is, shall we say, merely interlaced. It is only tight when gooped with caulking, no teflon possible here,
You would be able to go under your boat, and if you hadn't used 5200, 4200 or 3200, theoretically easily unscrew the thruhull. That's why you have so many threads, male and female, makes it unlikely. A NPT straight thread seacock is really held on by the flange fastenings. Believe it or not. Somebody tell me if I'm wrong.
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