Got the heads off today, no signs of water coming from another place. Plugs missing was it I would say, plug threads are rusty. Really thinking about bringing it back to life. Where can I get a manual for the engine and any pitfalls I should be looking out for. It's a 1986 3.7 170
you can get a manual from Mercruiser
https://publications.brunswick.com/
99% probability it was the head gasket that went first
the 470 was not a highly sought after motor
its sort of the platypus of motors - Mercruiser aluminum block and head, GM flywheel, mercury damper, custom crank, custom cam, ford FE iron head, ford FE rods and pistons.
gaskets, especially the head gasket has to be a mercury marine gasket. its the only head gasket that works
the floating bores and aluminum block with the iron head means that any change in temperature so much as a 10 degree rise in temperature, the head gasket would spit out.
hence the heat exchanger being required.
initially the 3" heat exchanger was offered, however the motor would still over-heat if a fish swam past the water pickup, so it was upgraded to a 4" to give it a bit more head room.
the cam snout was not hardened and as a result, the water pump seals would wear grooves in the cam snout and the water pump would start leaking coolant. make sure your has been fixed with the speedy sleaves.
the stock charging system is an outboard motor stator on the back side of the damper. the voltage regulator is what normally fails, and is really expensive, so an alternator conversion is normally done.
if your going to rebuild the motor, my suggestion is as follows:
get an edelbrock aluminum head for the 460 and have it modified by drilling the large hole for the 90 degree elbow
have a laser-cut block bore girdle made. it only needs to be 4 or 5mm plate. have it tack welded in place.
have the bores checked prior to any machine work. the most you can go is 0.030 over. after that there is not enough meat left and you may need to run sleeves.
