Read my piston if you can

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RiverRat2 said:
Is this a set up???? You got me gun shy you slick devil,,,,,, :buttkick: :buttkick: :buttkick:
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LOL, no River, I wouldn't pull two fast ones in one week. That piston is in a 660 that came in this week. Only 2 years old and somehow got to pre-igniting from a hot spot in the cylinder. Had the correct spark plug in it and the owner has 4 other Stihls using the same mix yet this one failed. The top inside of the cylinder is burnt half way around as well leaving bright shiny pits, now not fit to be used. Head man at Stihl told me if I was to merely replace the piston that pitted cylinder top would create hot spots and burn a hole through the next one. As to what exactly caused the hot spot that started that saw to pre-ignite remains a mystery, the other 4 saws are running fine.
 
Tom, I've seen it a number of times in automotive applications. The hot spot was on the piston due to detonation. I would blame the fuel first. Although he has other saws runnig the same fuel. HE RAN THAT ONE TILL THE HOLE WENT ALL THE WAY THROUGH.

That saw was not running well before the hole was done.

Fred
 
Tom,
I have several detonation failures in my collection. They can all be related to lean running.

This one (deto1) had 2 lean faults, an air leak at the clutch seal, and trash in the fuel filter screen in the carb. Lean means less fuel. So what is in the combustion chamber burns faster, so the flame front speed becomes very fast. This sets up detonation, where some of the fuel lights off after the plug has fired, and two or more flame fronts collide. It often happens at the same place, near or under the plug, so you may get a hole hammered through the top of the piston, like this one (deto2).

The location of the hole in your piston kinda makes me think that maybe someone used a rigid screw in piston stop that caused a fracture in the top of the piston, where it is thin, and then it blew out later. That is why you should use a plastic stop that gets pinched in the squish area, where the piston is stronger.
Did you find any lean faults on the saw?
Just curious. I have yet to prove that low octane or timing can kill a saw, and I have tried to do it. No luck!
 
Mr. said:
Tom, I've seen it a number of times in automotive applications. The hot spot was on the piston due to detonation. I would blame the fuel first. Although he has other saws runnig the same fuel. HE RAN THAT ONE TILL THE HOLE WENT ALL THE WAY THROUGH.

That saw was not running well before the hole was done.

Fred
Click to expand...

Fred I agree with ya, it had to be running poorly. The thing that gets me is what he told me it did. According to him he was sawing up a big ash tree. He said he sat the saw down for a few minutes to clear some brush. He went to restart and nothing, as if to say the hole melted through while it was not running. He brought it to me already with the jug off so its puzzling. It does have the correct plug in it and he says his other saws are using the same mix and running fine so beats me. My main man at Stihl pointed to fuel too but its odd the others are running fine on the same mix, so the feller tells me anyway. I'm gonna toss it back together with new jug and piston Friday so hopefully it doesn't happen again. Of note the piston showed hardly no wear at all except for the hole. The cylinder looked perfect too cept for burning around the top inside edge.
 
Stihl #1 said:
Tom,
I have several detonation failures in my collection. They can all be related to lean running.

This one (deto1) had 2 lean faults, an air leak at the clutch seal, and trash in the fuel filter screen in the carb. Lean means less fuel. So what is in the combustion chamber burns faster, so the flame front speed becomes very fast. This sets up detonation, where some of the fuel lights off after the plug has fired, and two or more flame fronts collide. It often happens at the same place, near or under the plug, so you may get a hole hammered through the top of the piston, like this one (deto2).

The location of the hole in your piston kinda makes me think that maybe someone used a rigid screw in piston stop that caused a fracture in the top of the piston, where it is thin, and then it blew out later. That is why you should use a plastic stop that gets pinched in the squish area, where the piston is stronger.
Did you find any lean faults on the saw?
Just curious. I have yet to prove that low octane or timing can kill a saw, and I have tried to do it. No luck!
Click to expand...

He brought that saw to me about 2 month ago to have the rpm's checked. It read 13,300 on my tach. I lowered it to 12,800. It may have well been running 300 over max since he had it, about 2 years, I don't know. He brought it in already tore apart. I'd like to have seen what that saw was doing when it let go. I'm kinda at the mercy of what he tells me. It is like I say, the very first piston I've seen burn straight through. I've seen many that look like they got hammered on but this is the first burn out I've seen. Kinda interesting I must admit...
 
Mr. said:
Tom, I've seen it a number of times in automotive applications. The hot spot was on the piston due to detonation. I would blame the fuel first. Although he has other saws runnig the same fuel. HE RAN THAT ONE TILL THE HOLE WENT ALL THE WAY THROUGH.

That saw was not running well before the hole was done.

Fred
Click to expand...

That is weird all right I couild understand it if he had a bunch of carbon buildup and it had raised the compression and his fuel chioce was 87 octane,,
but even so detonation usually peels the outer edge of the piston down??????? that one is a weird one all right,,,, I havent read the whole post,,, I was curious as to what the pest or lake thought about it??? :monkey:
 
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RiverRat2 said:
That is weird all right I couild understand it if he had a bunch of carbon buildup and it had raised the compression and his fuel chioce was 87 octane,,
but even so then pre ignition or detonation usually peels the outer edge of the piston down??????? that one is a weird one all right,,,, I havent read the whole post,,, I was curious as to what the pest or lake thought about it??? :monkey:
Click to expand...

Good point River. I'm hoping Lake takes a good look at it and gives his thoughts on it. I'm 100% honest when I say its a mystery to me as to why this saw did this. I couldn't check it for air leaks or anything because it was brought in already with the jug off. When I relayed everything the owner told me to my man at Stihl his answer to me as to why it happened was a simple I don't know based on what we were told. Ole Lake will give me some clues to look for I'm sure...
 
I hate when that happens!!!!!!!!!!

THALL10326 said:
Fred I agree with ya, it had to be running poorly. The thing that gets me is what he told me it did. According to him he was sawing up a big ash tree. He said he sat the saw down for a few minutes to clear some brush. He went to restart and nothing, as if to say the hole melted through while it was not running. He brought it to me already with the jug off so its puzzling. It does have the correct plug in it and he says his other saws are using the same mix and running fine so beats me. My main man at Stihl pointed to fuel too but its odd the others are running fine on the same mix, so the feller tells me anyway. I'm gonna toss it back together with new jug and piston Friday so hopefully it doesn't happen again. Of note the piston showed hardly no wear at all except for the hole. The cylinder looked perfect too cept for burning around the top inside edge.
Click to expand...


That is when someone has torn it down before I can cheeck it out!!!!!!!!
Tom once you get it built,,, I would give it a thorough going over for leaks, pressure and vacuum,, and while you got her tore down I'd
pull the carb apart check the metering diaphram/fuel pump and check that inlet screen for debris it may be really fine particulite matter but as previously stated,,,somethin caused it to get toasty!!!!!!!!!!!!
 
RiverRat2 said:
That is when someone has torn it down before I can cheeck it out!!!!!!!!
Tom once you get it built,,, I would give it a thorough going over for leaks, pressure and vacuum,, and while you got her tore down I'd
pull the carb apart check the metering diaphram/fuel pump and check that inlet screen for debris it may be really fine particulite matter but as previously stated,,,somethin caused it to get toasty!!!!!!!!!!!!
Click to expand...


True, I was like hmmmmmmmmmmmmmm when I seen it. I'll give it a good check over after I toss it back together. Last thing I want is seeing that saw again a year from now..
 
THALL10326 said:
Not to derail his thread or anything but thats the first piston I've ever seen burn clean through. Pre-ignition or as some call it, dieseling, from a hot spot on top of the inside of the cylinder was the culprit. She sure got hot about it didn't she....
Click to expand...

Looks to me like someone used the wrong piston stop and cracked the top... then ran it.. Can't use screw-in pison stop with angled plugs!



Edit.. I see Stihl1 also thinks that...
 
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Lakeside53 said:
Looks to me like someone used the wrong piston stoi and cracked the top... then ran it..
Click to expand...


I pondered on that and had my suspicions but going by what he tells me its hard to tell what happen. The inside top of the cylinder does have some hefty burns in it so I don't know what to think other than just toss the huzzy back together. I was quite amazed, it is the first piston I ever seen with a hole in it. Holy piston that wasn't blessed for sure,lol.
 
ms310 original sealant

I am original owner and the saw has never been broken down before, so the red sealant must be original
 
I'l put a new piston in it and start from there. You'll soon know if the coil is stuck in full advance or something else screwy. I assume he didn't advance his ignition?
 

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