I'm running SBC pumping out 425Hp and have never had any problems until I let the boat sit for 2 months and a valve stuck. I replaced all the valves and put the motor back together and all of a sudden got oil in my water.(the engine ran great) I have the heads in the shop getting checked but they said they look good. Any ideas. They are sportsman II iron heads and an aluminum edelbrock performer intake. All looks good. Any ideas??

Also if I have to replace the block, is there a marine block or is any 4 bolt main block good.

Intake manifold gasket? What type of exhaust? Riser gasket leaking?

I had bad exhaust manifolds last year. Worked fine with the garden hose test, but the crack opened when hot.

Would a leaky manifold cause the oil to turn milky before your eyes while running. When I changed the oil and flushed the pan I ran it with the valve covers off and you could see it turn milky.
That said, I checked my risers and the gasket between the flange does have a very small leak only while under pressure. Reversion wouildn't make it to the oil pan that fast......would it?

Yes, that is exactly what mine did. The oil would almost immediately turn to chocolate milkshake, and within a few minutes it would be vanilla milkshake. It also took out a few valves with it.

Your manifold/riser gasket/mating surfaces could do it as well. In fact, all I know is my problem had something to do with the exhaust. I bought Revolutions and never looked back. To me the extra $500 more for them is worth the piece of mind, because they don't have that infernal wet joint between manifold and riser. They use a hose.

I highly recommend them. They will be on my next boat as well.

You may have saved me a lot of down time and wasted money. I was looking for a new block but now it will be a new exhaust with a dry joint. If that solves it I'll be estatic.

Intake manifold gasket would be my first suspect.

Were the intake bolts tight when you removed it?

All the gaskets were good and all bolts were sealed with ARP thread sealer. I changed the intake gasket twice and even used silicone gasket maker the second time to rule out any leaks.

Did you check the intake itself? Johno (you met him) had one fail that was practically new, with disasterous results.