Stihl 066 Piston Wear Question

[Wood Doctor]

Last month I replaced a worn out top end with a new cylinder and piston kit. Compression was then sitting on 160 psi. The owner and operator ran it through less than three tanks of mixed fuel. With a 30" bar on board and while cutting big cottonwood, the engine lost power and stopped suddenly earlier this past week. Compression is now at 75 psi. I just inspected all the other top end components. Wrist pin, bearing, and clips are all normal. Here is the intake side:

And, here is the exhaust side:

So, does anybody have any idea what caused this failure in such short order? I'd like to avoid a repeat performance when I rebuild it again. TIA.

 

[jmester]

Looks like you have a bad pto side crank bearing. Or a lean seize hard to tell exactly by the pics.

 

[Thommo]

Looks like a piece of metal has jammed into the intake side and pushed the piston across so it scored the exhaust side. I would be thoroughly checking bearings to see if a bearing cage is breaking up.

 

[jmester]

Did you rebuild the carb?

 

[Totembear]

Maybe a bearing coming apart, maybe a bur on the ports, or something got in the intake side. There is a good service manual Stihl put out about piston failure.

 

[Wood Doctor]

Did you rebuild the carb?

No. The carb seemed to be OK. However, if it was set lean at WOT, perhaps that set up the failure? Good carbs running the engine lean can ruin the piston, but this seems incredibly fast.

 

[jmester]

Check the main bearings. The plastic cages can only take so much. Have seen a few 660 with bad pto bearings. I would rebuild the carb and retune.

 

[angelo c]

What's the chain look like ?
I've seen a few well rebuilt saws get returned toasted because the over paid operator decided to undersharpen and over load a good saw....

 

[Wood Doctor]

What's the chain look like ?
I've seen a few well rebuilt saws get returned toasted because the over paid operator decided to undersharpen and over load a good saw....

Chain is sharp and little, if any, wear -- 063 3/8 semi-chisel. Bar is also in good shape. No signs of burn anywhere.

 

[Wood Doctor]

I've now shown the piston to several other mechanics. The consensus is that the saw was straight gassed. The owner says that he filled the tank only twice. However, the saw was stored where other employees could have used it and then filled the tank with straight gas without him knowing about it. Then when he used it once again, the saw was likely running on straight gas until it stopped dead in its tracks. I guess it can happen to anyone. He's sick about it.

Regardless, I'm going to rebuild the top end for him. Santa will be busy in the workshop.

 

[jmester]

Are you going to check the crank bearings. Did you pressure and vacuum test the crankcase after you rebuilt the first time.

 

[Wood Doctor]

Are you going to check the crank bearings? Did you pressure and vacuum test the crankcase after you rebuilt the first time?

Yes, all was fine. Usually if you have an air leak there, the engine will not idle down and instead sit on about 4000 to 4500 RPM while trying to idle. This saw ran fine at all speeds after the rebuild.

 

[jmester]

Ok just wanted to make sure.

 

[Wood Doctor]

Ok just wanted to make sure.

Thank you. The problem could also have been made worse up by the carb running lean at the high end. However, it usually loses cutting power when that happens. This was not reported to me.

 

[jmester]

I think you are on track with straight gas. For me I would just double check the pto side bearing and seal for that is where most of the scoring is.

 

[Chainsaw Jim]

Photo bucket sucks.
It looks like you may have more than one problem here. The air filter may be letting dust through, the bearings could be just bad enough to act up at wot, pin bearing cage may have let loose to cause the nasty gouge and compromised the oil seals, the carb running lean from needing rebuilt, rakers on the chain not at the correct depth.

 

[blsnelling]

That intake side damage does not look like it came from straight gas. That looks like damage from a foreign object.

 

[jughead500]

check the pto side seal

 

[DSS]

No way was that straight gassed. Something was in there that shouldn't have been.

 

[jughead500]

also check and see if that could possibly be a model with shielded bearings.
cant tell much from the small pictures but to me on my one run in on a shielded bearing 036 i bought from nmurph for my buddy steve.the shields cause build up of gunk in the seal lip causing the lip to pooch out and suck air.
with most of the damage being on the pto side i would say the culprit would be there.