You pretty much have the same engine that I have. I’m guessing you have the same problem I had - that the rear seal in the internal water pump is bad. To keep coolant from getting into the engine at the end of the camshaft, there’s a weep hole that lets it drip into the bilge (the water pump is driven by the very front end of the cam.
It’s a pretty big pain to get into it to fix it but if you’re a fair mechanic and a glutton for punishment, you can do it. On mine, the shaft was galled and I had to find a repair sleeve that press fit over the galled section of the shaft. I screwed up the first seal I put in and had to do it all over.
I have manuals but our resident boat mechanic, Spareparts saved me from shooting myself when I was fighting some pesky locating pins that wouldn’t let me get the cover back on. He a very generous guy with his knowledge and will probably chime in when you have problems.
If you find the weep hole and it’s the problem (probably won’t leak until it gets warm enough to build some pressure) and don’t find a manual, I’ll see if I can find the applicable pages and scan & email them to you. I have also had fair luck finding blow up diagrams and even manual sections by Googling around for them, so you should try that first.
Regarding the rusty bolts, I had that problem and it diminished a lot when I fixed the coolant leak so the engine compartment wasn’t so humid. I think the humidity builds up and then condenses when the engine cools off, rusting the bare metal bolts. That’s speculation but it fits pretty well.
It’s a good wintertime project.
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 Scott, Portland, OR '85 V-20 I/O, Merc 170
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