I just replaced the complete sanitation system on my 1981 367. I decided not to plumb the raw discharge and to plug the seacock with a bronze plug. The seacock is stuck open and has been that way since a bought the boat 2 seasons ago. It also has a pvc elbow installed with some sort of sealant that I can't get out. I tried heating the housing up but all it does is cause to PVC to get soft and bend when I try to unscrew it. The pvc elbow is pretty much destroyed now. Looks like the seacock is original and the bronze housing has a top piece that might be screwed into the bottom part. This is in the worst place to get to. One idea I have is to cut the top of the pvc elbow off and add heat towards the threads and try to pry the rest of it out and when clean up the threads. Is there a better way to do this? Is there a way to remove the seacock and work on it outside the boat? I'm not going anywhere until I fix this hole in the boat.
Mike
I can't help too much, but we just went through replacing/rebuilding our seacocks, including the one you are working on under the starboard settee bench. This one is a real bad word to get any leverage on. Ours is a Perko valve and I did some of the work and the yard guys did some. I rebuilt it with a new ball and seats, sand blasted the housing and replaced the shaft seals. It was quite a bit of time and may not be worth it if you value your time too highly. Parts were only about $75 bucks, but especially if it is as frozen up as you describe, a new valve may be in order because cleaning the old one up is such a big job and it may not be salvageable if the flange where the seat goes is bad. A new valve runs around a half a boat buck. The new ball was plastic whereas the old ball was chrome plated bronze.
If the sealant is 3M 5200, it is really hard to overcome, but there is a solvent for it http://www.westmarine.com/metal-protectants/yacht-chandlers--debond-polyurethane-adhesive-spray-cleaner--12222295. Remove what you can and give it some time. There is a special tool to hold the little ears from the outside while someone else turns the valve from the inside. You will strip the ears if you try to put too much torque on them. You can also make a tool if you are handy. Maybe a socket of the right size with a slot ground into the wall with a die grinder? Compass marine has an article on it. http://www.pbase.com/mainecruising/replacing_thruhulls
I have had some limited luck cutting the sealant between the valve flange and the base using a mat knife blade and forcing it into the joint with a punch or screwdriver. A razor blade might work too. Thin and sharp and watch your fingers. Of you can work it all around the flange, there is a smaller arm to resist you when you try to rotate the valve, and it lets the solvent get to the sealant.
It is possible to rebuild a Perko valve without removing it from the boat by unscrewing the top part that retains the ball, removing the ball, then the lower seat and replacing the seats and the ball and cleaning up the casting, but you are working through that hole and bad words will be said. I wouldn't recommend in my situation, but it might work for you. I tried removing the tail pipe on mine and gave up, too frozen.
In order to get the ball out, you need to be able to rotate the valve to the closed position because there is a slot in the ball that keys to the handle. On a recommendation from the yard guys, I put it back together with white lithium grease. Scotch bright (I used purple) only on the seat flanges. Because the new seat has to seal against that surface.
Some valves are bolted to the hull. That I believe is what most people recommend these days, but not how our boat was built.
Good luck.
Seacock is working. Liberal soaking with PB Blaster got it moving. As for the PVC elbow: cut it off just above the seacock, then heated it up when a heat gun until it was soft, and popped it out with a screwdriver. Have ordered a bronze plug.
Mike