@ Fotoman, read, heed, and need. All wrapped up in a very informative piece.
Replacing Thru-Hulls and Seacocks Photo Gallery by Compass Marine How To at pbase.com
The MOST important thing to learn is the definitions of pipe thread.
NPS is: National Pipethread Straight. l l
NPT is: National Pipethread Tapered. \ /
Using these simple diagrams you can see the two surfaces do not mate for more than a couple threads. Bad JuJu on a boat.
You can screw NPT into NPS, BUt not the other way around. However, the connection (as described by Compass marine is extremely low quality, and risky for underwater thru hulls. In reality only two threads are fully engaged when doing this type of connection.
The mentioned Groco adapter (or actual Groco or Conbraco adapter WITH integrated valve) is FAR superior. I used the adapter WITH valve, since it eliminates one more threaded connection between the spud and the valve.
To bring the discussion back to the reality, do you REALLY want an imperfect partially threaded connection between the valve and the ocean, depending on it's integrity as an inferior connection? Don't just screw on a valve to the mushroom spud.
Make sure you read all the way to page 2! It's enlightening.