OK then, I like your backbone. Somebody not afraid to speak out. You're wrong but I admire your gall.
Help with 066 failure analysis (pics)
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OK then, I like your backbone. Somebody not afraid to speak out. You're wrong but I admire your gall.
IMHO after the piston started going South and things heated up, a lot happened to the top of the piston.
Hard to tell on the piston if it was pieces of the piston getting up there to make those marks or if it was preignition or detonation?
By then, it didn't matter, the piston wasn't any good any way?
Hard to tell on the piston if it was pieces of the piston getting up there to make those marks or if it was preignition or detonation?
By then, it didn't matter, the piston wasn't any good any way?
tony_a
New Member
question
You said initially that the motor had been rebuilt. Do you know if that is a genuine stihl piston and cylinder or an after market part.
Tony
You said initially that the motor had been rebuilt. Do you know if that is a genuine stihl piston and cylinder or an after market part.
Tony
LegDeLimber
Addicted to ArboristSite
Forgive my going tangential here.
Haven't been hands on with any recent saws, But have seen a couple of Catalytic mufflered
trimmers with really hot looking piston zones at the exhaust port area....
been cases where the cat/refractory material is just inside the muffler inlet (visible mesh)
and *seems* to throw a lot of heat back on the exhaust side of piston.
...and yes, there was always ethanol involved.
Haven't been hands on with any recent saws, But have seen a couple of Catalytic mufflered
trimmers with really hot looking piston zones at the exhaust port area....
been cases where the cat/refractory material is just inside the muffler inlet (visible mesh)
and *seems* to throw a lot of heat back on the exhaust side of piston.
...and yes, there was always ethanol involved.
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That erosion on the lead edge of the piston crown looks like slow creepy corrosion from having sat for a while with water in the muffler. The white death could have been what started the ball rolling on that failure …..Or a textbook casting flaw……..
Tuned too lean
low octane fuel
Plug too high heat range
To much alcohol in fuel
Lean condition caused by vac leak
Or any combination of the above.
low octane fuel
Plug too high heat range
To much alcohol in fuel
Lean condition caused by vac leak
Or any combination of the above.
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First of all dump that batch of gas in the mower.Then rip the saw down rebuild it with a new piston and crank seals and vac/pressure test it.Check the heat range of the spark plug and the tuning of the carb when you get it running.
Some more information. The saw was completely rebuilt one year prior, with all genuine Stihl parts at the Stihl dealer (new piston, cylinder, and carb kit). He ran it for about one year until this happened. Said he was in a log when it lost power and died. Said it would not restart. I know him pretty well and I highly doubt straight gas. I do suspect ethanol because we are mandated E10 for all 87 octane. He uses the saw for big work felling and also in a jig at his house for ripping. Maybe this will help figure it out, maybe not. I just don't want to dump a bunch of money in it and not know the problem. I checked the bearings and the seem good. I plan on doing crank seals, carb kit, piston, cylinder, base gasket, fuel line, and impulse line if I fix it.
You’re right, figuring out what happened is a good idea… fixing something with out fixing the cause is senseless.
Let’s take the issue of ethanol…
Just ignore statements like “ethanol is not the cause of this lol. I run ethanol in my saw daily, so do most other people. probably straight gassed.
Most other people? Well not ‘round here. Besides, that’s like saying “Drinking and driving won’t get you arrested, I’ve been drinking and driving for over 30 years. Probably flipped the cop off.”
What about detonation?
Well the damage from detonation in a two-cycle engine running 12k+ RPM is a lot different than what you’ll see in a four-cycle running 7k Rpm. Detonation is the spontaneous “explosion” of the fuel after the spark plug fires… initiated from heat and pressure. First, the spark plug of a high-performance two-cycle (like a chainsaw) is normally centered over the piston, and the controlled burn happens relatively fast… remember it’s turning over 12k. Detonation is most likely to initiate on the exhaust side, were heat is the highest. The damage from detonation happens three-fold… at the point of “explosion”, at the point where the two flame fronts collide, and at the point where the shock wave rebounds against something. Because of the fast combustion rate in a high-performance two-cycle engine, the flame front from detonation may only travel one millimeter or less before it collides with the combustion flame front. All of the damage is localized at the edge of the piston, where it “blows away” that edge or corner… and shortly after that it gets to the ring… and shortly after that… (and that’s surely what I see in your picture).
Octane?
A lot of guys are gonna’ argue with me, but 87 octane fuel is horrible for any high-performance engine… including chainsaws. And something most guys don’t understand is that the addition of mix oil lowers the octane of the fuel even more (yeah, that’s right; oil has a lower octane rating than gas). “Octane” is a measure of the fuel’s resistance to “detonation”… once ignited (such as by a spark plug) it burns at pretty darn close to the same rate regardless of octane rating. Yeah, I know a lot of guys run 87 octane fuel… but that’s just like the “drinking and driving” thing above… they just ain’t been unlucky yet. And remember, the octane rating is minimum... meaning it may be higher, but never lower (supposedly). You may go for months, or years, actually buying 88, or 89 octane at the "advertized" 87 level, but suddenly you get 87... and then you lean on the saw...
So if you put it all together…
Low octane fuel, more likely to detonate.
Ethanol-blended fuel causing an engine to run hotter (even if tuned).
More heat, more likely to initiate detonation.
And then what happens when you get fuel with more than the “advertized” 10% ethanol?
At least by running a 91-92 octane fuel you give yourself a bit of margin for error… but at 87 octane you’re already on the edge.
Was ethanol the root cause? Who knows for sure… but I’d bet it was a huge contributing factor at the least.
Let’s take the issue of ethanol…
Just ignore statements like “ethanol is not the cause of this lol. I run ethanol in my saw daily, so do most other people. probably straight gassed.
Most other people? Well not ‘round here. Besides, that’s like saying “Drinking and driving won’t get you arrested, I’ve been drinking and driving for over 30 years. Probably flipped the cop off.”
What about detonation?
Well the damage from detonation in a two-cycle engine running 12k+ RPM is a lot different than what you’ll see in a four-cycle running 7k Rpm. Detonation is the spontaneous “explosion” of the fuel after the spark plug fires… initiated from heat and pressure. First, the spark plug of a high-performance two-cycle (like a chainsaw) is normally centered over the piston, and the controlled burn happens relatively fast… remember it’s turning over 12k. Detonation is most likely to initiate on the exhaust side, were heat is the highest. The damage from detonation happens three-fold… at the point of “explosion”, at the point where the two flame fronts collide, and at the point where the shock wave rebounds against something. Because of the fast combustion rate in a high-performance two-cycle engine, the flame front from detonation may only travel one millimeter or less before it collides with the combustion flame front. All of the damage is localized at the edge of the piston, where it “blows away” that edge or corner… and shortly after that it gets to the ring… and shortly after that… (and that’s surely what I see in your picture).
Octane?
A lot of guys are gonna’ argue with me, but 87 octane fuel is horrible for any high-performance engine… including chainsaws. And something most guys don’t understand is that the addition of mix oil lowers the octane of the fuel even more (yeah, that’s right; oil has a lower octane rating than gas). “Octane” is a measure of the fuel’s resistance to “detonation”… once ignited (such as by a spark plug) it burns at pretty darn close to the same rate regardless of octane rating. Yeah, I know a lot of guys run 87 octane fuel… but that’s just like the “drinking and driving” thing above… they just ain’t been unlucky yet. And remember, the octane rating is minimum... meaning it may be higher, but never lower (supposedly). You may go for months, or years, actually buying 88, or 89 octane at the "advertized" 87 level, but suddenly you get 87... and then you lean on the saw...
So if you put it all together…
Low octane fuel, more likely to detonate.
Ethanol-blended fuel causing an engine to run hotter (even if tuned).
More heat, more likely to initiate detonation.
And then what happens when you get fuel with more than the “advertized” 10% ethanol?
At least by running a 91-92 octane fuel you give yourself a bit of margin for error… but at 87 octane you’re already on the edge.
Was ethanol the root cause? Who knows for sure… but I’d bet it was a huge contributing factor at the least.
And a little lean on the carb setting.
Ethanol in and of itself will not cause that. The saw was not tuned properly. He should also be running 32:1 when milling. Milling generates a lot more heat in a saw than most falling/bucking. It should also be tuned much richer when milling, so rich that it will barely clean up in the cut.
Be cool to rig some type of cyl temp gauge on them milling saws.
Actually, running more oil in the mix will cause the engine to run hotter... for five reasons.
Oil sinks less heat than gasoline as it travels through the crankcase and cylinder.
Oil burns at a higher temperature than gasoline.
Oil lowers the octane level of the fuel mix.
Even when properly adjusted for the mix, the carb will be delivering less gasoline then if it was using a mix with less oil.
Using more oil in the mix will rob power from the engine, causing it to work harder.
Staight gassing would cause damage all the around the piston and cylinder. Steve
Good list, also bad fuel line, plugged filter, clogged tank vent. Steve
Actually, the opposite has been proven true, due to better ring sealing. Ask around and see how many experienced millers are running 50:1.
the westspartan
Addicted to ArboristSite
I like the auto-tune/m-tronic saws for milling as they compensate for the heat by adjusting tune through the cut! So far so good with my 441m-tronic. It will be nice when the bigger displacement saws get this technology, as a 70cc saw has a limit in what it likes to mill.
Me,
Many thousands of BF of hardwoods with a 066. Always mixed with Spectro at 50:1. Stopped running pump gas when they added MBTE, went to 100LL.
Almost all my saws still have OEM machine marks on the pistons, lots are 20+ years
old, but only get used a bit last - 5 years.
Been using the same/oil ratio since 1973.
Why change what works?
Haven't seen but (apparently) the exh side. I'd expect the exhaust side to experience damage first & worst anyhow.
Looks mainly like it's been dry of lube for some while under heavy load. Aka straight-gassing.
The operator was clearly not able to hear what the saw was telling him- a rookie. Straight-gassing not at all unlikely for such operators. One such fellow asked me back when "what does it mean when smoke is coming out of the saw everywhere?" Duh.
Sticking with straight-gassing by complete idjit. With a dose of leanness. And dirty cooling fins.
Lean or straight gas , all damage is on ex side
Saw was running too rich before carnage.
Saw was running too rich before carnage.
