Tragedy averted

[EFSM]

A customer brought in a CS-620 Echo that wasn't running right. After changing the weird fuel that was in it, it ran very well, but made a strange knocking noise at idle. So I pulled the muffler to inspect. I'm not sure how much longer the rings would have taken this. I had ported this saw a couple of years ago, when the original crankshaft bearings failed and I stuck the piston and cylinder onto a CS-600 crankcase with the old mega-advancing coil. Should I be worried about the cause of this or chalk it up to very high hours?

EDIT: I realized in looking back at sales history on the 620 piston that I replaced this one last year due to a broken crown. This coil advances far enough for the saw to rev 500-700 rpm's more than a 620 coil. Is that part of the problem?

 

[HumBurner]

Is that sludge between the rings or a chunk missing??

 

[EFSM]

That's a chunk of the land missing.

 

[Powerstroke Cowboy]

That's a chunk of the land missing.

That's no man's land now.


Good save! I wish they all were found before the blow.

 

[markds2]

That type of damage is normally caused by over-revving for exteneded periods of time which causes ring flutter:

 

[HumBurner]

That type of damage is normally caused by over-revving for exteneded periods of time which causes ring flutter:
View attachment 438597

Is there a solid definition of over-revving? I haven't heard that phrase before.

WOT with no load?

Edit:

Or is it just running too lean/at too high of rpms for extended periods? Idling too high?

 

[markds2]

Is there a solid definition of over-revving? I haven't heard that phrase before.

WOT with no load?

Edit:

Or is it just running too lean/at too high of rpms for extended periods? Idling too high?

I understood it was WOT with no load. I've got a 500i here at the moment with the same damage along with a ring locating pin hammered out too. I believe that saw was used for a lot of limbing.

 

[Motorka]

ring locating pin hammered out too

Cold high revs ?

 

[davidwyby]

I could see a heavy trigger finger limbing doing that. Or a ported saw in small wood with high rakers.

 

[markds2]

ring locating pin hammered out too

Cold high revs ?

I'm not sure, it's a very high hour forestry saw. I'll try getting pics tonight. I've had a couple of 661c's through with same broken out piston ring land on the exhaust side, all were very high hour forestry saws.

 

[EFSM]

The Echo under discussion is a limbing/bucking saw, not a felling saw. Due to the rarity of the coil on it, I don't think many 620's, even the hardest ported ones, run so fast.

 

[HumBurner]

I'm not sure, it's a very high hour forestry saw. I'll try getting pics tonight. I've had a couple of 661c's through with same broken out piston ring land on the exhaust side, all were very high hour forestry saws.

I can see where a hard life of sprout-cutting/thinning small stuff, especially with a dull chain or high rakers could lead to that, but my crewboss's 346s have thousands of hours on them under the same conditions, with only one or two having been rerung a time or two.


Edit: and he runs 50:1

 

[davidwyby]

Exh port shape?

 

[EFSM]

my crewboss's 346s have thousands of hours on them under the same conditions, with only one or two having been rerung a time or two.

A smaller bore/piston will stress a piston differently. Due to a tighter radius, the ring lands should have higher torsion resistance and be less prone to breaking.

 

[markds2]

661c on the left and 500i on the right. Both stock saws.

 

[Motorka]

Piston engine failure analysis.