Piston Failure Cause

[Cedar Row]

After reading other threads about piston failure caused by running lean, I wonder if the damage I mentioned earlier in this thread was caused by a leaking fuel line instead of broken muffler debris. The fuel line had essentially disintegrated and was leaking. A hole in the fuel line could cause a lean situation.

 

[drf255]

I cleaned up this piston, stricktly to use in the bore for Degreeing the saw.

On examination with the piston upside down on the bore, the side clearance was around 060, and the skirt was 060 thick on unbroken intake side and worn to .015-.020 on the broken exhaust side.

So clearly, excessive piston side clearance with rocking of the piston had to contribute to it's demise.

 

[Icehouse]

Here's a couple pictures of a piston that came from an 036 I rebuilt about a year ago then gave to a friend. He kept telling me it was running worse all the time then one day he said it didn't sound right, so I took apart and found this. This was an after market piston and cylinder, from where I don't remember, ran real strong for a couple months before I gave it to him. Looks like there has been a lot of piston slap or slop, after putting a mic on piston and cylinder find about .025 difference, just a damn poor p&c package. I'll check better next time before installing.

 

[Wood Doctor]

I just ran into a similar failure on an 066 Mag. The bottom skirt on the exhaust side is torn to shreds, and the rings are worn out and fused. The intake side is clean. Only took the part-time operator about two months to do it. The jug hardly has a scratch. I'll post a Pics if you want to see yet another.

What I have to ask is what happens to all the fragments that get ripped off the skirt? Is there enough heat in there to vaporize them as the piston flies apart? I can't find any shrapnel anywhere in the case and the spark screen on the muffler is still intact. Note that the 066 also has a small casing inside the muffler right at the exhaust port.

 

[nmurph]

The piston bits and bites didn't evaporate, they are there somewhere. Any time some sort of metal gets loose inside the saw, the seals come out and everything gets flushed WELL with high pressure from a water hose.

 

[drf255]

So pull the seals on my saw?

 

[nmurph]

I'm a bit anal about the inside of a saw. Other's fuss over getting the last speck of dirt off a saw after they've cut a cookie, I fuss over getting the last speck of metal out of the inside of a saw. I guess it's all about priorities.

 

[Wood Doctor]

Looks like we have to tear this whole thing down and do a complete clean up. You also have to wonder especially in my case if the fragments from the previous piston and rings contributed to the failure this time. These may be building up and creating a snowball effect. So, without a complete clean up of the seals and bearings, we are walking on thin ice with any subsequent replacements.

Note to Lone Wolf: This is the same 066 Mag saw we were discussing in our conversation. Turns out the engine was blown away in less than two months after a complete rebuild. I/we thought it was a fuel/carb problem and it turns out that the piston and rings were shot. I never dreamed it would fail that fast--possible in less than two hours of run time. Gasp!

 

[Wood Doctor]

I cleaned up this piston, stricktly to use in the bore for Degreeing the saw.

On examination with the piston upside down on the bore, the side clearance was around 060, and the skirt was 060 thick on unbroken intake side and worn to .015-.020 on the broken exhaust side.

So clearly, excessive piston side clearance with rocking of the piston had to contribute to it's demise.

I have to add another question. OP and I both have ring and skirt failure. Which one occurred first and could either have caused the other? Or, could both have occurred simultaneously, induced by cylinder play?

In my case, the jug is the original Stihl that I preserved, and then I added a new piston and rings when the originals gave up after a dozen years or so. This jug remains in good shape, or is it? I have to wonder now that this second set of piston and rings have failed along with skirt damage. The first failure had no skirt damage, simply worn out rings and no compression.