Rebuilding failed 660

[Frisian]

I got 660 with cracked crankshaft in the clutch area.
Looks like genius that assembled the saw forgot to put crank seal on the clutch side.
Upon inspection I see that piston pin got blue, which is expected as all lubrication was likely escaping via crank bearing that had no seal.
Piston rings and cylinder look OK in my opinion.
The question I have, would you replace the cylinder as well?
The saw has very low hours of use and failed because careless assembly.

 

[Marine-piper]

If the cylinder is in good shape I wouldn't worry about it, rings and piston look great.

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[Mad Professor]

Looks to me like they forgot clutch was left hand thread

 

[stihl86]

Looks to me like they forgot clutch was left hand thread

It does look that way. I wonder what other surprises are lurking. Might be a good idea to check everything, if the saw is worth it. Taking a good look at the flywheel.

 

[Bob Hedgecutter]

Would it have run without the PTO seal in place, well enough or long enough to "blue" the piston?

 

[Short timer]

That crankshaft did not fail because the seal was missing. And the seal wasn’t missing when the crankshaft failed if it was running.

 

[RED-85-Z51]

Somebody ugga dugga'd the holy hell out of that clutch

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[Bob Hedgecutter]

That crankshaft did not fail because the seal was missing. And the seal wasn’t missing when the crankshaft failed.

That would be the way I look at it as well.
Muppet take down, righty tighty lefty loosey uh oh snappy.
Okay how do I get the rest of the roddy thing out? Pick out the seal........ um this looks too hard...... sell it?

 

[Short timer]

That would be the way I look at it as well.
Muppet take down, righty tighty lefty loosey uh oh snappy.
Okay how do I get the rest of the roddy thing out? Pick out the seal........ um this looks too hard...... sell it?

No way that cylinder and piston would be in good shape if it was running without a crankshaft seal when the crank failed.

Ain't happening.

 

[Bob Hedgecutter]

Or how about- the saw body and oiler area are just far too clean.
So saw was being rebuilt- oil seal was missed and somebody leaned on the crank real good with a rattle gun set for truck lug nuts while putting the clutch back on.
Is it an OEM crank even? Maybe the Cinga ones have a lesser breaking strain?

 

[Short timer]

Normally there will be an old partial fracture that is visible with stains and then the clean new fracture at the point of failure. This happened all at one time from what I can see in the pics, as in overtightening the clutch.

 

[Short timer]

Or how about- the saw body and oiler area are just far too clean.
So saw was being rebuilt- oil seal was missed and somebody leaned on the crank real good with a rattle gun set for truck lug nuts while putting the clutch back on.
Is it an OEM crank even? Maybe the Cinga ones have a lesser breaking strain?

This is the most likely. I’d say you nailed it.

 

[Frisian]

Looks to me like they forgot clutch was left hand thread

I just put the clutch like that to take picture.

 

[Frisian]

Thank you all for responding.
I ran the saw when failed. The crank broke maybe 20 min after start. I was getting some gas fume smell and couldn’t figure out where it was coming from, then in a bit chain got loose and after taking cover off I noticed that clutch assembly broke off.

This is kit saw that was assembled by someone who was probably in a hurry or after some beer .
I intend rebuilding it with Farmertec parts. Purchased new crank, piston, bearing and piston pin.
Was just wondering if cylinder could be reused, it doesn’t look ruined from the inside.
I probably will take chance, it is not that difficult to swap it if it will not run well.

 

[Frisian]

Normally there will be an old partial fracture that is visible with stains and then the clean new fracture at the point of failure. This happened all at one time from what I can see in the pics, as in overtightening the clutch.

You think clutch was over tightened?

 

[rogue60]

OEM Stihl ms660 they do snap not very common but it can happen.
The Chinese just copied the ms660 crank along with the weakness in that area of the crank along with subpar materials no doubt being chinese made.

 

[Short timer]

You think clutch was over tightened?

Anything is just a wild guess now that you’ve added more details, but I’ll stand by my statement that the crank didn’t break because the seal was missing. And with the seal missing I don’t even see how the saw could be running long enough, if at all, to break a crank.

There needs to be case compression for the mix to transfer into the cylinder, and case vacuum to draw the mix into the case. The carburetor wouldn’t even pump fuel from the tank without a sealed case. Everything about a 2stroke engine is dependent on a sealed case. If you were to vac/press test the engine right now, it‘d read zero no matter how fast you pumped the handle. I suppose even though it being a massive leak, if you could get the engine started, the momentum of the piston could output more pressure and vacuum than can escape through the leak because it’s a big enough air pump, but I don’t see how you would have enough carb adjustment for the saw not to meltdown the first time you cut with it. She’d be screaming for mercy at WOT and then go silent. I’d like to know how many turns your H screw is turned out. You’re going to have to retune after repairs anyway so if you can count the turns it takes to bottom out the H needle (clockwise) and report back, that would be great.

How hard was it to start? Did you have to squirt mix in the carb throat for it to start?

The question I have, would you replace the cylinder as well?

Was just wondering if cylinder could be reused, it doesn’t look ruined from the inside.

If it’s not scored then there’s absolutely no reason the P&C needs to be replaced. Pictures of the exhaust side of piston and cylinder wall would help make that determination.

Were you cutting with the saw for this “20 minutes” or just trying to get it to run? You also said this saw “has very low hours of use”. Are these your “hours“ running and cutting with it with no PTO side seal?

We are getting into the territory of 2+1 does not equal 3. As in, the saw possibly being in service for “hours“, a missing crank seal, and a usable P&C.

Fug it, why deal with crank seals, just richen her up.

 

[Short timer]

Is this the 660 with the broken crank with no PTO side seal and a P&C that aren’t destroyed?

https://www.arboristsite.com/commun...-with-your-chainsaw-today.335072/post-7409364

 

[Short timer]

OEM Stihl ms660 they do snap not very common but it can happen.
The Chinese just copied the ms660 crank along with the weakness in that area of the crank along with subpar materials no doubt being chinese made.
View attachment 864985

This is what I was talking about in my other post. Normally when a shaft cracks while in use, it holds on for awhile before complete failure. Unless it’s a dramatic event like a sudden stoppage, impact, etc.

The original crack is older looking than the fresh crack when complete failure occurs. I’m not doubting what the OP is saying, but it’s not the norm, probably because of inferior China junk.

His break was instantaneous and yours was a more common failure, as in it cracked, you had no idea it was cracked, kept running the saw and eventually it completely failed. Pictures are deceiving, but this looks like what happened with yours, if that half moon area isn’t a raised nub casting a shadow.

 

[drf255]

Pretty sure the PTO bearing is in backwards. Those Farmertec 660 kits come with the seals already installed in the bearings, along with a fair amount of metal dust.

The proper install for the PTO bearing is the plastic cage towards the crank and the deeper portion out towards the oil pump (which is used to set overall bearing depth). Without that installation, there is nowhere to install and retain the seal. If the saw ran at all, it’s possible the seal is intact sled towards the crank.

Those saws are a rabbit hole of fixes. Good luck.