496 H.O. Problems
 

One of my engines with 240 hours started making a loud banging noise. The heads were taken off and it had a rocker arm broke in half and a bent pushrod. Also the valve spring could be pushed down by hand.I don't have a clue how this could happen, except for defective parts from the factory. Anybody have any ideas??

How do you run the boat typically? Sticks to the dash or cruise? Air it out often? Break a drive and over rev it? Rev it in neutral?

How did that piston look like ? could be that the piston broke in the wrist pin area so it could move up some and hit the valve ,therefore broke the rocker arm and bent the pushrod (just think loud ) might be different then

The piston is fine. I cruise 3500 to 4000,the engines have less then 2 hours run time over 4000 rpm's.The drive is ok,but I'm sure I hit the rev limiter a few times.

try to turn the crank back and forth and see if the piston moves all the time ,if so the problem is not there ,if that valve spring is that weak like you said ,the piston will hit the valve as the pressure on the valve is not enough ,think at 4000 rpm this could be the case

There is a bolt and a set screw top of the valve stem, rocker. If it gets loose the rocker arm will start flapping, brake, stuck, rod bent etc. Let's hope all you need is a set of new valves, rod and a rocker. Occasionally, it's a good idea to check the torque on those bolts.

The weak valve spring stands out as the problem to me. The springs inability to keep sufficient tension on the valve, rocker arm, push rod could allow the push rod to lift out of the pockets on the lifter and rocker arm. It then gets caught on the higher sides of the pockets creating, in effect, a longer pushrod on the next downstroke from the cam. This pushes the valve too far, further compressing the spring and letting the piston hit the valve hard breaking parts. I would check or replace all valve springs while it is apart so it does not happen again

The heads were reconditioned and all the springs were replaced with Crane heavy duty springs.

Should not have that problem again. Good luck.

I had this exact same condition happen to me with my 2004 Velocity 290SC that I purchased brand new. At about 150 hours (hard hours I will confess) I popped the push rod right thru the seat of the rocker arm. About $1K later, I was out running her and popped another rod thru the rocker on the other side of the engine!!! I was Flippin' pissed at this point and called Teagues Custom Marine to explain my dilemma. They immediately told me that that Mercury Marine has issued a bulletin on this exact dillio and should be helpful with rectifying the situation. (And that they were!! They paid for the whole repair plus reimbursed me for the first repair that my f..king idiot Boat mechanic/service center billed me $1K for) .....THEY will never see one of my boats in there shop...EVER!
Needless to say, I'm pretty happy I reached out to Bob Teague's group.......they saved me a lot of money and aggrevation.
After the second fix, I traded the Velocity for my SSR......except I opted for 525's vs. 496's and couldn't be happier about my decision:evilb:

To me.... at first glance, it sounds like your situation is a lot like what I experienced.......
Give Mercury Marine a call and run this by them and feel free to drop my name and boat as a reference point!
Good Luck!