Saturday was a beautiful day on the water, for about an hour anyway! Was running from one end of the lake to the other and thought I would wind it up alittle, at around 4800 RPM "20 seconds" I felt the boat jerk and starboard motor was dropping RPM and stalled. Engine temp. was around 160 and oil pressure 45 PSI. I stopped right away and raised the engine hatch and noticed what seemed to be a small puff of steam belch out of the flame arrestor. I attemped to restart engine and had an uneven cranking RPM along with rattle every revolution. I checked the oil then and it was about 6 inches over full then found the coolant reservoir to empty.
I had read about weak pistons on the 496's on modified engines, has their been much of a problem on bone stockers with 40 hours? I am assuming the piston came apart and went through the cylinder wall. Are their certain year engines better than others?

Anything can break,40 hours is it under warranty?
If so just take it to a mercury dealer.If it did break a piston they will more than likely chang the whole engine.
Water in the oil and cylinders is either a catastrophic failure like piston puts hole in block.Or an exhaust issue and possible hydro lock.

I think you are on the right track....have heard of injectors going bad or clogged..leaning out and killing pistons !!

If a piston broke up it would have gone out with a bang and would not crank over. Pull the plugs. See if one is damaged.

It happened so fast that the antifreeze pouring in the motor never mixed to a milkshake...I have not torn down yet, but with the un-even cranking RPM and deep rattle every cranking revolution it doesnt sound good...I am going to pull plugs and used a friends bore scope and find out....

If it is under warranty you are in luck.

If not... Pull your plugs. If one is chewed up you may as well pull that motor because the show is over. Check the #7 cylinder, a few other guys lost #7 including myself. More than likely a piston imploded then the connecting rod punched a hole in the cylinder wall and sucked the anti-freeze out. Since it sucked in oil & anti freeze at 4000RPM you probably bent a few rods as well.

Here is what it will look like....

Sounds to me like you had a coolant leak in it and hydrolocked it. dont ask me how I know... pull the plugs and drain all the oil and fluid and then get fresh oil back in it, even run it for a couple minutes with the belt off to get oil circulating, it could be a plethora of things from a gasket either head or intake to something more serious.

I just pulled the plugs and number 1 was smash!

Any idea what is causing the failure....Cast or forged pistons?

They are cast... you might have been lucky and NOT poked a hole in the cylinder or block. If so, you can rebuild with forged or maybe do Raylar kits or something.

I cant tell from your picture. Is that plug melted or a piece of aluminum stuck in there?