65hp Johnson / fractured piston skirt

[jasonh]

I recently got some great advice here and had one other question - my previous motor was a 65hp Johnson (1974 I think). Prior to the boating season, it was fully tuned up with new impeller, lower oil, carb kits, plugs, etc... The compression was a solid 110-115 on all three cylinders and ran perfect the entire summer until coming home one night I felt a THUD! and a sudden drop in power. I was a good quarter mile from the ramp, so I pressed on and actually made it back.

Upon inspection, there was a hole in the crankcase large enough to see the crank and connecting rod. I tore the entire powerhead down and realized part of the piston skirt had broken off. The piston was still connected to the rod and the rotating assembly moved ok. The curious thing was that the piston fracture appeared somewhat old (not shiny like the case and had light carbon deposits), like maybe the piece had broken off on a previous trip and just then jiggled into place where the flying crank blasted it through the case.

Anyway, my question is this - being as how there was absolutely no indication something was amiss prior to it failing, what sort of thing could have led to that happening? I used quality Johnson oil at a 50:1 mix, it appeared to pump water well, and while I sometimes ran it WOT, I did not keep it there nor was it WOT when it happened. I'd like to avoid this happening with my 1976 85hp, so any tip about how that could have happened or what to periodically check would be appreciated.

Thanks!

 

[Dhadley]

Re: 65hp Johnson / fractured piston skirt

Can you get some pics? If so and you have trouble posting them, send them to me at dhadley50@aol.com and I'll get them up for you. I think this may be one of theose times that a pic is worth......well, you know.

 

[iwombat]

Re: 65hp Johnson / fractured piston skirt

I've had that happen in cars before and there seems to be two probable causes.

1) Excessive wear in the bore. Enough to cause lots of slapping at the bottom of the piston.

2) Running lean and hot. Enough to overheat the piston and cause the metal to work-harden and fail.

 

[Dhadley]

Re: 65hp Johnson / fractured piston skirt

Here's the pic --

 

[jasonh]

Re: 65hp Johnson / fractured piston skirt

My thanks to Dhaley for posting the picture. This is admittedly not a very informative picture and I am trying to locate the images of the piston that broke and of the crankcase hole. The above picture was taken approximately 12 hours after it failed, so the rust you see is not that old. Also, the other two pistons were fine upon inspection. No signs of pitting, scouring, or other damage.

 

[ezeke]

Re: 65hp Johnson / fractured piston skirt

I can see the carbon has built up on the piston tops and would have also gotten to the rings. That's enough to force a ring out enough to catch and break the piston.

It is hard to imagine the perfect tuning that would prevent that, but a periodic decarb would certainly not hurt.

I've had fewer problems with pistons since I switched to ashless synthetic oils. Edit:: running hot did not contribute to that carbon, but running cold sure would.

 

[jasonh]

Re: 65hp Johnson / fractured piston skirt

I've read some favorable reviews of Seafoam Deep Creep as a method of removing built up carbon deposits, so I believe I will try that on my 85hp in case it's gunked up.

Thank you all for your assistance and tips, what a great site.

 

[iwombat]

Re: 65hp Johnson / fractured piston skirt

A stuck and then broken ring is a good theory, and a problem particular to two-strokes.

 

[Solittle]

Re: 65hp Johnson / fractured piston skirt

It should be interesting to see what the piston looks like as you did not mention if a ring was broken. Also I wonder how water got into the cyl.

 

[ezeke]

Re: 65hp Johnson / fractured piston skirt

Something either broke or melted, and did a tap dance on the face of that piston.

 

[G DANE]

Re: 65hp Johnson / fractured piston skirt

Sorry to say but that rust is older that 12 hours. If there were oil left in the bore it wouldnt rust like that, so I guess either your lower carb clocked totally or more likely it ran with a blown headgasket, letting water in mid and lower, and steam cleaning oil from piston and bores. Running like that would wear pistonskirt out rapidly. I cant htink of any way a big piece of a piston skirt can hide anywhere in a running motor, there istn room for it.

 

[jasonh]

Re: 65hp Johnson / fractured piston skirt

The piston that failed did in fact have the rings still intact. That was the first thing I looked for after finally banging the piston out of the cylinder. The sides of the piston were scratched and scoured and the top surface was pitted and degraded. I think G Dane may have called it -water had been present in there for a while. I have no idea if it was a gasket or carb though. It was a Friday night when it happened. I did not know what happened when the engine suddenly lost power, I just knew that if I shut it down I'd never get it started again. If I remember correctly, the hole in the case went through the water jacket. While it was running those 15 minutes, I imagine water was everywhere in the lower cylinder.

The next morning I got up, backed it in my garage, and within a couple hours had the motor taken apart. That was when I took the above picture.

 

[Silvertip]

Re: 65hp Johnson / fractured piston skirt

Many years ago I had that exact thing happen on a Johnny triple. Perfectly sound engine, perfect state of tune. It chucked a piece of piston skirt into the side of the block but the hole it created had a flap which was able to be pushed back into the block and successfully welded. There was no other evidence of damage and I an exact cause of the failure was never determined. Things can break and this was just one of those things. Bore was in perfect shape, no out of round taper was within spec. Installed a new piston and rerung the others. The engine lived on.