MS362 toasted piston

[MAF143]

A friend of a friend brought a saw over to me this evening to have me look at it. Seemed to be locked up. The guy he had loaned it to had taken it to a dealer when it quit and they told him it threw a rod. We popped the muffler off and poured the chunks out of it and looked at the piston, it looked GONE... I saw why once the cylinder came off.

Anyway I told him I would pull it apart and see what I could do. It for sure needs a P&C and the bottom end needs cleaned out real good. I'll change the seals and feel the bearings real good to see if they have any tix. It's about 10 years old he said and it is not M-tronic. He said it was about 10 years old and he cuts about 8-10 cord a year.

After getting the big chunks out of the crankcase it rotates fine, but I will clean it real good before messing with it much more to see if it will have to be split and the bearings replaced.

I'm new to fixing saws so I was wondering if anyone could ID what may have caused a failure like this. Not that it really matters, but he's curious to know if it was something that was going to happen anyway or if the guy that borrowed it maybe oopsied...
just and unfortunate piston failure?
straight gassed?
too lean?

would other pix help tell what caused this mess. This is the exhaust side of the piston. The intake is scored some from chunks, but not like this side.

 

[Motorka]

Show the top of the piston. Look like detonation.

 

[MAF143]

Didn't notice anything on the top, but I'm a rookie.





I had chunks of chinese crankshaft bearing go up through the 391 I used to have and it didn't look nearly this bad. I was able to save the P&C on that one and was able to clean it up and put OEM bearings in it and it ran great with a little tweaking...

 

[Ketchup]

What did the wrist pin bearings look like? Any slop in the con rod? The exhaust skirt hit something at full speed. Probably the con rod or a foreign object.

 

[MAF143]

The wrist pin felt OK and was tight, no slop, same with the rod bottom bearing, feels good, just side to side and tight.

I couldn't see any signs of impact on the rod, but again, I'm a rookie at this kind of damage. Crank bearings feel tight.

The exhaust port looks carboned up and has some aluminum smeared on the top edge and a little on the bottom but looks normal other than that.

It didn't feel like the muffler bolts had been taken off recently when I broke them loose. The dealer that looked at it must have just pulled the starter cord and said "rod, buy a new saw."



Exhaust side of con rod


Intake side of rod

 

[sledneck22]

The rod shows signs of heat at the bottom bearing as well as the inner wrist pin. Better check that crank over really well.
As far as diagnosing the failure.... based on those two observations, heat was involved. Maybe not the only reason but it attributed to it.

 

[MAF143]

That makes as much sense as anything. The chain was dull, there was a lot of built up gunk all over the saw, and the air filter was extremely covered in saw dust powder...

 

[sledneck22]

I apologize if my picture looks like a set of balls on the connecting rod but......
The arrow points to the obvious heat markings.
But the red circles with yellow lines in them kind of appear like the rod twisted. The rod itself still looks fairly perpendicular to the crank but that could just be the "angle of the dangle" here.
I'm not well versed in these 362s but i find it strange one side of the rod has these concave "dents" in the raceway where the intake side does not.
I would be skeptical of this crankshaft if it was my rebuild.

 

[MAF143]

Some of the other Stihls I've worked on have those waves in the rod bearing. I assumed they were to allow a little more lube into the bearings. The "dents" are on both sides and look to be cast or forged in by design. I know my 391 had the same "waves" on it's rod big end.

I will clean it up and check it out very well. This far in it only makes sense.


LOL (he said "balls") LOL

 

[sledneck22]

I assumed they were to allow a little more lube into the bearings.

That would make sense. I haven't paid enough attention to the ones I've had apart or the perception of the picture just exaggerates the openings.
I chuckled when I drew that and figured, why change it now, it gets the point across.

 

[MAF143]

We all need a chuckle at least a few times a day... And balls seem to bring out the chuckles...

 

[huskihl]

The blueing could be from heating the crank for assembly purposes. I’ve seen it on some new saws and I’ve seen no evidence of it on older saws. Doesn’t appear to be any rhyme nor reason as to the consistency

 

[Lightning Performance]

The blueing could be from heating the crank for assembly purposes. I’ve seen it on some new saws and I’ve seen no evidence of it on older saws. Doesn’t appear to be any rhyme nor reason as to the consistency

What he said ^

Put seals in it and vac check the motor after assembly. Go through the carb good and make sure it sticks the tune when you wake it up. If not you got carb troubles. Cut the limiter s anyways.

 

[Maintenance Chief]

Dull chain? Nuff said. I pick saws out of the scrap yards around me and the one consistency is a dull chain on a roasted saw. Often over looked as the main cause of a scorched piston.

 

[Lightning Performance]

Dull chain? Nuff said. I pick saws out of the scrap yards around me and the one consistency is a dull chain on a roasted saw. Often over looked as the main cause of a scorched piston.

That I do not agree with or get. Dull or sharp should not cook a power head imo.

Dragging down the RPM to make it cut will burn one up, out or however you see it. That is pure lack of air flow. Nothing to do with the chain. I've watch a guy burn up a 361 with sharp 28" 8pin running factory carb settings with a fresh loop full comp just off the roll. But... he knew better and it was bad gas in his opinion. Watched it happen.

 

[Maintenance Chief]

That I do not agree with or get. Dull or sharp should not cook a power head imo.

Dragging down the RPM to make it cut will burn one up, out or however you see it. That is pure lack of air flow. Nothing to do with the chain. I've watch a guy burn up a 361 with sharp 28" 8pin running factory carb settings with a fresh loop full comp just off the roll. But... he knew better and it was bad gas in his opinion. Watched it happen.

Usually its the inexperienced cutter that is burning through cuts in 2-3 minutes that should take 30 seconds with a sharp chain. Its like just piss revving the saw for hours during use .
I appreciate your responses ,but I have seen many examples of poorly maintained saws with roached pistons that ran flawlessly after the p/c were changed and passed a pressure check.
The dull chain doesn't just slam the upper rpms it also introduces super fine particles into the air filter and wears the operator out which leads to more bad decisions.

 

[Lightning Performance]

Oh wel

Usually its the inexperienced cutter that is burning through cuts in 2-3 minutes that should take 30 seconds with a sharp chain. Its like just piss revving the saw for hours during use .
I appreciate your responses ,but I have seen many examples of poorly maintained saws with roached pistons that ran flawlessly after the p/c were changed and passed a pressure check.
The dull chain doesn't just slam the upper rpms it also introduces super fine particles into the air filter and wears the operator out which leads to more bad decisions.

Just adding in fines, bad or stale gas is just par for the course.
Most stuff around here is straight gas by the owner. They never admit it though. Gave up asking years ago.

 

[Lightning Performance]

Usually its the inexperienced cutter that is burning through cuts in 2-3 minutes that should take 30 seconds with a sharp chain. Its like just piss revving the saw for hours during use .
I appreciate your responses ,but I have seen many examples of poorly maintained saws with roached pistons that ran flawlessly after the p/c were changed and passed a pressure check.
The dull chain doesn't just slam the upper rpms it also introduces super fine particles into the air filter and wears the operator out which leads to more bad decisions.

I'd bet you get some really nice bars on saws lol.

 

[huskyboy]

Stihl themselves say that it’s not supposed to be blued. They were kind enough to answer my question, I appreciate that they did that for me. Anyone can ask them questions via email on there tech support page. @MAF143 I would definitely hesitate to reuse that crank with this knowledge in mind. It probably lost its temper and will fail sooner rather than later.

 

[Lightning Performance]

Stihl themselves say that it’s not supposed to be blued. They were kind enough to answer my question, I appreciate that they did that for me. Anyone can ask them questions via email on there tech support page. View attachment 301946

Chrysler/Marnier said the same thing.
Most of us think that is BS. We saw blueing on so many new or near new parts in small engines I'm sure the factory will never ever admit it. They sell new parts everyday.

Seen it on brand new replacement cranks.