rust on top of cylinders 488 merc io
 

fellas, finally freed up 1984 488 merc. let it soak for ten days. pulled heads and found 2-rear cylinders with rust on valves and pistons, nasty looking. head gasket appears to be an automotive one but no evidence of jumping water jackets etc. question how did the water get into those top end cylinders and valves? also had 2 bent rods. did the water get sucked up from exhaust and perhaps the internal flapper valve stuck.? alum. block and cast iron head. water got into heads, no water in oil. oil looks good. mech says will rebuild but i need to know cause of water intrusion. thinking water backed up from exhaust when shut off last time out comments appreciated thank you! richard

How long was it sitting? I know on one of my old motorcycles (Yamaha 500 DOHC) that sat for over 5 years the motor siezed.. turns out that the exhaust valve was in the open position when it was shut off.. As the motor sat in the sun it heated up, and at night it cooled...over and over.. that heating and cooling drew a lot of water vapor (condensation) into the motor over the years.. causing it to eventually rust. There was really nothing wrong with the motor, just sitting was enough to rust it up. I don't know if the same is your case, but it's something to think about. :head:

water in cylinder heads
 

how did rust in cylinders get there? boat in storage enclosed for 6 mos. motor siezed. spent ten days soaking and was able to turn over with wrench. took heads off, gasket ok 2 bent rods and 2 cyls with rust. did water come from backing up thru risers? richard

riser gasket, or the riser itself. replace both(use teh merc riser gasket) if you rebuild the engine. Check hte manifold as well. There is not an automotive head gasket available for that engine as its a mercruiser engine, never used in a car.

spareparts thanks
 

thank you for the info, mon. will take a hard look at riser and manifold, will probably buy new ones, for it is difficult to establish the inner wall condition of both parts unless you can advise on propper way to check both, would be glad to attempt this before ordering. again thank you for the time and suggestions. richard:hi::sun:

flip the riser over and look inside the exhaust passage, poke around with a screw driver and see if it pokes thru the inner wall. The manifold is a little tougher, first inspect the gasket surface where the riser meets. Secondly, pull the manifold, block off the hose fittings with some spare hose. level the manifold on a table, fill the water jacket till it almost comes thru at the riser flange, let it sit over night and see if the level drops. If you want to, you can make a block off plate and pressure test the manifold

Id follow Spares advice! It sounds like a bad riser or leaking gasket. Check Mercruiser Parts Express, thats where I got mine from. Its an original replacement.

look into buying a used motor . there are some good running motors available now at days. you will need more parts then you think once you start on this project. good luck! Break Out Another Thousand spells boat.:hi: