Piston is destroyed but what causes this? (images)

[1966-60HorseJohnson]

Hey all,

I have a V4, 1966 60 hp Johnson 2-stroke. When replacing the spark plugs I noticed that in 3 cylinders (#1-3) the spark plug was a little black and the cylinders were dry, and in the #4 cylinder the spark plug was clean and the cylinder was wet. I removed the cylinder head cover today to find that the piston in #4 is pretty much destroyed, and the cylinder head was damaged as well. I retracted the piston to check the cylinder and oddly enough it looks ok; no scoring, grooves or anything that I can see. The pics are of the damaged piston, the good piston on that side, and the cylinder head cover.

I realize this will take some major work, but my question is: what causes damage like this?

 

[woody66912]

Re: Piston is destroyed but what causes this? (images)

Hey Ole Come look at this. Holy crap man. I would tend to think a half-plugged main jet?.

 

[F_R]

Re: Piston is destroyed but what causes this? (images)

Nah, that isn't pre-ignition. That was caused by something breaking and it swallowed the broken parts. Could be a piston ring, could be a bearing cage, or a couple other things. Only a tear-down and inspection can tell.

Before somebody tries to tell you otherwise, it wasn't the oil either. But it could have been ingesting water, which caused bearing failure. Like I said, a tear-down is in order.

I've seen lots of similar damage in my 24 years in the shop. Hey, it fed my family.

 

[boobie]

Re: Piston is destroyed but what causes this? (images)

After 46 yrs of running something was bound to break.

 

[orbanp]

Re: Piston is destroyed but what causes this? (images)

Could it have been pre-ignition detonation? That tends to destroy the landings on the pistons.
Was ignition timing correct for that cylinder?
It is hard to believe that there is no damage to the cylinder! The picture is not too good on that. Run your finger across the cylinder surface, that is quite sensitive for scratches and other imperfections.

Good luck, Peter

 

[seahorse5]

Re: Piston is destroyed but what causes this? (images)

Looks like detonation damage from either a hot-spot or severe overheating or running lean for some reason. If there was an air leak into the intake or crankcase from a bad gasket or a split hose, that could do it. If the motor had a history of spitting or backfiring at idle, that would be a give away.

 

[yorab]

Re: Piston is destroyed but what causes this? (images)

Post back with lots of pics of your failure analysis.