Stihl 066 Piston Wear Question

[Wood Doctor]

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

Yes, and it could have been a small chunk of the ring that broke off. I checked the crankcase prior to the rebuild and did a complete clean out. Nothing was inside that crankcase to cause this. A small fragment or two would likely not have heavily scored the exhaust side like this and burned out the rings dead flat to the side of the piston. Excessive heat did that.

 

[blsnelling]

Yes, heat did the exhaust side, but I think something else is going on with the intake side. What is the black beside the score mark? You mentioned a chunk of broken ring. I don't follow. I don't see where the ring is broken on this piston.

 

[AKDoug]

Lean seize on exhaust side, crowded the intake side and caused that scoring (opposite of the exhaust scoring). If it was bearings coming apart you would typically see it in the areas between the pin boss and the top of the piston because the junk travels up through the transfers. It's not junk from the intake because that part of the piston isn't open to the intake port.

 

[Wood Doctor]

Yes, heat did the exhaust side, but I think something else is going on with the intake side. What is the black beside the score mark? You mentioned a chunk of broken ring. I don't follow. I don't see where the ring is broken on this piston.

I don't see any missing piece from the rings either, now that I have looked it over carefully. I could have missed a small fragment that was still inside the crank case prior to the installation. That small remnant could have caused the intake side scar. Perhaps the cylinder lost a small particle as well once the heat seize started.

I have to point the finger at straight gassing because it happened so quickly. I doubt the engine got through three tanks of fuel. The first tank was the fuel mix that I supplied, which was OK because I used that fill-up to tune the carb. After that, the fuel was likely all straight or some sort of lean mixture.

I have recommended to the owner that he bike lock the saw to one of his other saws so that nobody else can use either of them without him knowing about it. They have had some trouble at this company before.

 

[jmssaws]

Seized and it don't have to have a air leak to do it, I c it all the time.
Gas and oil problem I'd say and I'm not saying straight gassed just a problem.

Bad gas and or oil will get more of them than a airleak will.