My son in law had my boat while I was overseas. This Mercruiser uses sea water for cooling and I don't think he rinsed it out afterwards. My first time taking the boat out this 4th of July, well the water hose broke off the gasket. The connecting piece looks like black plastic but I was told plastic could not get that corroded and it must be some type of coded aluminum. Anyway, I think it's 3/4" but I couldn't get a 3/4 simlar replacement in there, seemed to small. I tried a 5/8th which was way too small. I couldn't find a block and dye in 3/4" which I think I might need. Doesn't anyone know if it is a 3/4" or not? I did buy a plastic 3/4 connector but it would not to in there, but I'm also wondering if it would get too hot for plastic.
Bob

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water hose broke off the gasket??? cant picture what your talking about. do you have a picture? could it be your thermostat housing is broken?

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I'm not a motor person, but the spot where four hoses comes out of on the front of the engine is where I would think the thermostat would be. Two of the hoses goe left and right to the center of each side of the engine and they are towards the bottom, I can't see them from the tom I can only feel it underneath. The hose clamps to a connector about 3" long which first much be screwed into the engine. This hole I think is 3/4" but I can't get a 3/4" fitting into it.

I could take a picture of it and email it to you, first time using this site so not sure I can post or send pictures in here.

Your describing the plastic threaded tee inlet to the exhaust manifold. You can remove the exhaust manifold, turn it upside down and clear the threads. The plastic may have failed from improper winterization or over-heating. There also is a drain plug for winterization, molded into the bottom of the tee fitting.