rattelling/ knocking boat engine

my dad has a '84 Imperial VC-230 with a 3.7L "488" inline 4.. I know it shares the cylinder head with a 460 Ford. but not much other info about it... water pump went on the lake one day and it got hot.. my dad brought the boat in on the kicker motor, but the big motor still ran fine. later replaced the WP.. took it out and it's as what he describes as a rattly knocking.. says it sounds like it's in the head but sounds like a spun main. also told me it sounds ok without much load on it.. kinda sounds like a valve rattle to me, but my dad said it all looks good and is tight.. any ideas?

what all will it take to put a SBC in this boat if the engine isn't worth fixing.. every resource and boat shop my dad talked to said to put in a 3.0L 181 4cyl as it's a bolt in, but this boat is close to 5,000 lbs. and stock 188 hp.. it gets on plane at about 25 mph and will run until 44 MPH or so ( depending on how rough the water is ) trimmed out all the way. I'm not sure 135 hp is going to even get it on plane

Matt, the 488 was Mercruisers attempt to build a 4 cylinder engine that had some guts, and the 460 head helped a lot toward that. However, they also had somewhat of a bad repution for not being reliable, and very expensive to repair. Parts are almost non existent today and very costly when you can find some. One of the worst parts of that motor was the alternator that they incorporated into the crank damper. It was constantly getting wet and going out, so some companies developed a kit to put a regular alternator on it, but that kit was in the $ 600 range as I remember. A big source of problems was the aluminum block and cast iron head. Gasket failures were common, and the aluminum required a sealed cooling system.

I haven't seen one of these in years, but IMO you would be spending more to repower the boat than it is worth. That is a hard thing to accept sometimes, but with the boat market down you can pick up a more modern boat and engine setup for less than you will spend fixing this one. 26 years for a boat is like having a car that is 100 years old.

It could be something simple, so I would do the normal stuff like compression and leak down tests and try to isolate where that noise is coming from. I would suspect the head gasket because of the overheating issue. I know it's hard to scrap a boat your Dad probably likes, but in boating it all comes down to dollars and cents (or dollars and sense :))

Oh, and check the exhaust manifold for internal leaks too. The plugs will show rust if it has failed.

Don