Help with 066 failure analysis (pics)

[coyote556]

Guys,

I was wondering if I could get some help. I have looked this thing over and at first I figured it was just a typical lean issue, but then I checked Madsen's piston failure pictures and it looks like the detonation photos.

This saw belongs to a friend and I and rebuilding it for him to help him out. He is not real "saw savvy" and is not familiar with carb adjustments. He had all of his work done at a local shop and this saw was rebuilt for him last year (new piston and cylinder). I still suspect a regular old lean out, but wanted second opinions.

Thanks

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[ChainsawmanXX]

That piston is toast.
The cylinder looks like it will clean up fine with some acid.

 

[coyote556]

I wish it would. Hard to see in the photo, but the cylinder has significant grooving on the exhaust side. No hope for it.

 

[DarkTimber]

Wahts that odd square looking chunck in the piston pic? Part of the piston or is it some other part out of the saw?

 

[coyote556]

Best I could tell it was part of the piston. It seemed to be aluminum.

 

[Tzed250]

Bad gas I'll betcha...

 

[ChainsawmanXX]

Imma have to go with Tzed, bad gas.

 

[coyote556]

You thinking too much ethanol?

 

[Tzed250]

You thinking too much ethanol?

I'm thinking too much water in the gas. Usually caused by the ethanol, but it can come from many sources.

 

[CTYank]

Damage looks a bit much for over-lean mixture.

I'm leaning toward straight-gassing. And forcing the cut until it gives up in a cloud of smoke.

 

[Nitroman]

You see the line of bright metal up on the squish over the exhaust? Metal transfer from the piston. She got hot indeed.

Check these photos of piston wash and take a good look at the top of the piston:
Google Image Result for http://www.totallyamaha.com/snowmobiles/aaTECH/Piston_Wash/jet_wash.jpg

Another:
Google Image Result for http://www.cpcracing.com/members/256083/uploaded/wordpic.gif

 

[BigMoneyGrip]

Looks like detonation to me. You can see where the piston started eroding and then the ring went south.

 

[Whitespider]

I was thinking just like BigMoneyGrip, that eroded, melted area looks like classic detonation to me.
The question is, was detonation the cause or just a symptom of something else? Straight-gassing should normally stop a two-cycle engine before detonation can do that kind of damage. Detonation could have started because of excessive heat, or the detonation could have generated the excessive heat (well, detonation always generates excessive heat, but was it the original cause of excessive heat?). A lean condition can get detonation started, so can a fuel too low in octane, so can an overly hot engine. An ethanol-blended fuel will cause an engine to run hotter... and if it was a low-octane ethanol-blend... and if he was unlucky enough to get a low-octane ethanol-blend, that contained more than 10% ethanol, which will cause a lean(er) condition, and cause an engine to run even hotter???

So many questions... so few answers... BUT...

Yeah, I'm bettin' on a fuel issue... maybe fuel delivery, probably the fuel itself.
And if'n he was running an ethanol-blend I'd lean even harder towards crappy ethanol-blended fuel as the culprit...
Like I've said before,"Running ethanol-blended fuel in a high RPM two-cycle engine is like playin' Russian Roulette with it... it ain't so much that you've been lucky so far, rather you just ain't been unlucky yet."

 

[rupedoggy]

Well I have taken apart 100s of saws and that is not detonation. Look at the piston TOP. That is where detonation starts doing damage. That is heat and lots of it= low oil, no oil, lean or air leak (makes lean). It is a wonder a saw can run long enough to do that much damage. The operator was ignoring all the saw was trying to tell him!

 

[kevin j]

detonation is essentially a fatigue failure when the small pockets of mixture ignite rapidly rather than a controlled burn. The top of the piston is real dark for me in the pic. It shows pock marks but not that many so it may just be the casting of the piston. Det usually looks very granular, or or tiny mouse nibbles damage (much like the side of the piston above the top ring) but it occurs on the top of the piston, not down the side like that, as that area is quenched/cooled and doesn't see combustion heat directly.

I go with the others: lack of lube and much heat and eventually tearing off bits of that top land.

 

[w8ye]

This didn't happen for no reason out of the blue as has already been said.

This is going to happen again unless the cause is found.

There were several hints that something was wrong before it wouldn't run anymore.

 

[BarkBuster20]

ethanol is not the cause of this lol. I run ethanol in my saw daily, so do most other people. probably straight gassed.

 

[sefh3]

Well it looks like there are multiple issues. I'm betting the operator started the saw and hit the trigger and never looked back. The saw never warmed up and got really hot quickly.

You have a sign of seizure (lean) from bad gas, not enough oil in the gas and user error.

I would, read my signature line of how to build a leak tester, build it, leak test the saw. The piston needs replaced but that cylinder might clean up. The grooves look like aluminum transfer from the piston.

 

[parrisw]

Something is going on with all that erosion, look on the crown above the top ring, see all those tiny holes? Seems to be very lean, way too hot, too low of squish?? I'd say there is more then just bad gas going on here.

 

[Whitespider]

...that is not detonation. Look at the piston TOP. That is where detonation starts doing damage.

Where does that ridiculous notion come from?

addendum: You're confusing pre-ignition with detonation.