TOPIC: Mercruiser 165 1972 Sea Ray 190 Cooling Water Leak

Engine has a serious cooling water leak. This is a raw water cooled engine. There is a steady stream of water from underneath and behind the exhaust manifold, but I can't see exactly where it is coming from. (see photo with red arrow) And, there is trickle of water around a head bolt. Engine seems to run fine, but enough water coming out I have to run the bilge pump.

Sounds like you'll be pulling the head.. The old Chevy sixes are like the small-block V-8's in that the holes for the cylinder head bolts are open to the block's water jackets.. While this isn't a problem when these engines are used in cars & trucks with antifreeze in the coolant to prevent corrosion, it becomes a problem on raw water cooled marine engines because the threaded ends of the head bolts will rust away over time.. I've pulled apart many old Chevy straight six and V-8 marine engines that only had two or three threads left on some of the cylinder head bolts..

IIRC, there are also a couple of small core plugs (freeze plugs) located in the manifold surface of those old four & six cylinder heads.. If the engine had a remanufactured head swapped on it at some point in the past, it's possible that the rebuilder used plain steel plugs instead of the brass ones required for marine engines..

Thanks for the info. I guess I really won't know until I start taking it apart. Oddly though the bolt that is leaky runs through an exhaust port. I will not get to this right away. I have to much on my plate right now.

Do yourself a favor and drain the water from the water jackets, this will prevent water from getting in places it shouldn't until you can get time to deal with it.. I'd also pull the spark plugs, squirt a small amount of oil in each cylinder and spin it over a few turns with the starter to stave off any rust forming from any water that may have entered the cylinders..

Good luck!!