OK, are these pictures of your current powerhead now?  Or are these pictures of your original powerhead?  You mention that you have good water flow but it is hot.  You also mentioned that you pulled off your cylinder head on your freshly rebuilt powerhead, do I have it right?  Well, trying to find that bleed hole by probing around inside those cooling passages seems near impossible.  A few thoughts.  You mention that the cylinder head and exhaust cover temp seemed “OK”, but the “block was very hot….”.   What part of the block is “very hot”?  Your comment about the exhaust hsg being scorching hot leads me back to our hunch about a bleed hole being plugged up that would normally cool that area of the exhaust hsg.  The question is just where the bleed hole should be located, which I am not sure about.  Your picture of the original powerhead shows that little bleed hole that leads into that rear passage on the exhaust housing.  I’m guessing the cooling water is sprayed out into the upper exhaust hsg through this cavity.  Once again “I’m guessing” is the operative word(s) here.  Needless to say, your powerhead is installed on the exhaust hsg now, so we have nothing to look at.

You have the cylinder head off now, so examine the gasket closely, look for any signs of leakage.  Look closely at the head and block as well, perhaps there is some imperfection in the castings creating a slight leak.  Be sure to replace the head gasket, don’t reuse the current one.    But again, my hunch is that this is an issue of that upper exhaust hsg not getting the cooling water it needs.

Frank, are you reading this?  Your thoughts please.

OK, at fleetwin’s request, I’ll jump in and look.  Seriously, I think y’all might be barking up the wrong tree.  The water passages in those motors are big enough to throw a screaming cat through.  Original post says it was steaming out the water discharge, I suggest you revisit the water pump, looking for exhaust leaks into the pump.  And of course the pump housing.

Posting a picture of a problem that I’ve seen.

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bob-d

These guys have much more experience than me. I just wanted to know did you use a drill motor on the driveshaft to see if you were pumping water? Also gettig the block halves vatted there is a chance a chunk of something has lodged blocking waterflow. I had a hard time with a Mercury 5 hp motor getting good water flow. Had it appart & shoved everything that would fit in every water port & around the cylinder water jacket before it would run at normal temps. It wasn’t vatted either, just sitting for who knows long & given up by others. Only other solution I could suggest is getting tubing that fits the water ports & trying to force water thru the passages & see if it flows out & where. Good luck, sounds  like you put time into it. I cheated & had a light sourse from surgery & a fiber optic cable very small & I could light up the bottom og the water jackets where the water ports were. I used plastic optical cable from a robotic assembly line.  I needed light way down inside a leg on another project & had some time on my hands.  Good luck!!

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bob-d