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I’ve had a Jet boat for about four years. I originally installed a Chevrolet for 496 GEN five rebuilt.This motor lasted for about two summers, I spent a few bearings I took that motor to a builder and he said he was three months out so I left at one with him and bought a new short block same exact one so I could swap them if needed. Within about two months I started getting water in the oil. The mechanic chased it with every gasket you could ask for. And every kind of gasket. Other motor was done so I told him put rebuilt in. Now inbetween all that I changed the water system with jet pump pop off valve -pressure relief v
, water pressure gauge , and thermostat With 2 dumps vs three. He gave up. I took freshly rebuilt (maybe 5 hours on it) to another guy.

EVERYONE IVE TOLD THIS TO (MECHANICS AND JET BOAT OWNERS) Have never heard of it

Both motor had a hole in number 1 cylinder. In fact if you put a dime on the same side wall in same cylinder, both holes would have fit in that dim.

Sent off motor for sleeve and Magnaflux. In the middle all of this I have had the heads checked twice - didn’t want to leave that out.

So used the boat once, twice,,,,,on the second time i went to start it after running perfect all day, no water in the oil,,,,,,it hydra locked sitting in the driveway overnight .Took out plug ran all water out and all the other steps.

So used it,,,ran great, with new oil, got to ramp and pulled plug—-good to go. Went home and put it driveway. next day checked oil and good to go. Ran for about 20 mins water in the oil,,,think it was already there just wasn’t enough at the time i checked. Came home and pulled the freeze plugs and drained block. Changed oil and ran great-no water in the oil.

OK i had to give the back round. To cut to the chase the motor will sometimes not all the time fill the cylinder if i dont take out freeze plugs. Started taking them out every time and Never filled in drivewaY. However the water in the oil would slowly happen after 3 or 4 runs but never on the water and always checked frequently.

HELP HELP HELP LOL
 

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What bore size is it? .060" over for a true 496? Some people lump the 4.25" stroke engines all into "496" when they sell them. If it was bored more than .060" on a Gen 5 block there could be an issue.

What "every kind of gasket" was used? Gen 5 was known to have issues if the proper head gasket is not used with MK IV heads. That's one of the reasons it got updated pretty quickly to Gen 6.

I have had to find an oil leak in a block by dropping the pan and using very mild garden hose pressure to the water inlets. Found a sleeve was leaking that way and replaced the block rather than trying to mess with the thing.
 

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What bore size is it? .060" over for a true 496? Some people lump the 4.25" stroke engines all into "496" when they sell them. If it was bored more than .060" on a Gen 5 block there could be an issue.

What "every kind of gasket" was used? Gen 5 was known to have issues if the proper head gasket is not used with MK IV heads. That's one of the reasons it got updated pretty quickly to Gen 6.

I have had to find an oil leak in a block by dropping the pan and using very mild garden hose pressure to the water inlets. Found a sleeve was leaking that way and replaced the block rather than trying to mess with the thing.
I’ve been told it’s a gen 4
 

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Another question? what cam are you running and also (as stated) what exhaust? It could be a serious case of reversion. M also what engine?
Clay smith cam
However it fills the pan with water when sitting in driveway. It will run all day , show a little milk, then over night put a couple quarts of water in the pan
 

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Clay smith cam
However it fills the pan with water when sitting in driveway. It will run all day , show a little milk, then over night put a couple quarts of water in the pan
As I said above, I located a leaking sleeve in a block by dropping the oil pan and pressurizing the water jackets. I suspect you have a problem with the block, you might be able to verify by dropping the water level to the water pump bosses and letting it sit and see if the pan still gets water in it, but if you haven't already done so, sounds like time to pull the engine and drop the pan. There may even be visual signs of the problem such as a rust trail.
 
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