Check your 496's for valvetrain noise

[Knot 4 Me]

If you have a newer 496 MAG or MAG HO I would suggest lifting your engine hatch and listen closely for a rocker arm making a bunch of racket. Merc has confirmed there is a problem with some 496 rocker arms not being properly heat treated. The arm ends up shedding metal as it wallows itself out the the stud. In the case I am familiar with Merc is not replacing the motor even though there was significant metal missing from the rocker arm and metal was present in the oil filter. Hopefully anyone else with a motor with these bad rocker arms can catch it early.

 

[MattBMiller]

Is there a series of serial numbers the problem is known to exist in??

 

[Knot 4 Me]

That I do not know. The motor in question is in a boat that was built in May of 2008. Problem was diagnosed earlier this year at about 75 - 80 hours of run time.