On my 1988 4.3 L there were installed four of these. The threads to the pet **** seem to be 20 threads per inch, according to my thread gauge, but I can't find a nut in the hardware store that will screw on to them. Closest is 1/2 inch X 20. My conclusion is that this must be a proprietary thread size. The plug that the pet **** screws into has NPT 1/4 inch X 17 threads for going into the block and manifold, the same as my ball valves.
I tried removing the 5/8 inch hexagon plug using an open end wrench, which just slid around the corners of the hex. Next I tried a 12 point deep well socket - also rounded the corners. This is when I thought that the red on the threads in the above picture was Loctite. So I heated the plug with a propane torch, and it still would not come loose. (Red Loctite is suppose to loosen up at 550 F).
Thanks to Bondo, who said that is was just thread sealer, and not Loctite, I decided that the way to go was use a deep well 5/8 inch 6 point socket( skinny spark plug socket), that grips the flats of the hex and not the sharp corners. My 3/8 inch drive ratchet from Craftsman was pretty short, so I brought a foot long section of pipe to the boat in case I needed to increase my leverage. Because of the rounded corners of the hex, I had to tap the deep well six point socket onto the plug, put on my Craftsman ratchet, and no go. Added the pipe and I was able to move it. 10 seconds later the plug was in my hand. Screwing in the ball valve with Teflon tape on the threads was easy. My ball valves used a 11/16 in wrench.
I installed all four and I don't think anything will be in the way that can inadvertently open the valve. The starboard side block drain was the hardest to get to, I had to move the inlet hose out of the way to get the socket on the plug. After installation, I left the manifold valves open. I did find that I had to tip the bow of the boat up to get all of the water to drain out of the manifold. Since I use pink antifreeze - poured down the fat water pump hose, I opened the block ball valves for a little bit to flush all of the water out of the valve and only pink antifreeze came out. This should prevent freezing as warned by Chuckles88. I also poured some antifreeze down the water inlet hose until it was pink going out my outdrive.
So you can tell I am not a mechanic (a real mechanic would have used a 6 point right from the git go and been done three days ago), but I have total respect for the knowledge and skills that they demonstrate, and share on this wonderful forum. Thanks to all the regular contributors.
Next summer I will let you know how they work on a vibrating engine.
and close this thread 
