A proper sea cock, in the strictest sense (was) a tapered plug valve with a
flange on the outboard side and internal threads for the throught hull.. The
flange would be bolted though to the outside with a fitted block on the
inside. The through hull was just that and provide little or no structural
strength the the installation..

Pipe threads and none tapered through hull threads are weak because of the
amount of base metal that is removed during the threading process.

Today, you will seldom find a true and proper sea cock. What you will find
is a through hull with a valve threaded on the end of the inboard end of the
threads.. Even this is not a proper installation because the threads in the
standard valve will be tapered while the through hull threads are straight..
When tightened up, only the first couple threads will be holding the
pressure and providing mechanical strength.

There is often the recommendation that you try standing on your through hull
valve to ensure that the through hull is still sound.. That may be an
indication of strength of the exposed threads, however straight threads into
a tapered valve thread really leads to a valve that could easily vibrate or
be accidently loosened.

Myself, I'm using the Forespar Marelon flanged Seacocks, with Marelon
through hulls.. (objective is to have all non-metalic/non-conductive
fittings below the waterline)

The flange bolts were a problem so I used flat head machine screws and
counter sunk the heads and epoxyed over them to prevent contact with the
water. I will have to keep an eye on these over time to see if there is any
evidence of water intrusion..


--
My opinion and experience. FWIW

Steve
s/v Good Intentions