WTF happened here?

[SEAM]

There's not much left of the intake-side skirt... but the bits and pieces I found were not excessively thin (no machining marks any more, though). Piston and cylinder are original Mahle parts.

This is not about a claim against me or someone else, and I will not re-use the crank and bearings (too much debris involved). I have a spare engine with good top end to make a running replacement.
I've never seen anything like this and just wanted to know what may have cause this. My experience with top end failures (although I do own and run quite a few saws, trimmers and the like) is limited to a number of saws friends brought to me to have a look at. Most of those were heat related (air leak or carburetor/fuel problem), some due to bearing failure, one caused by a piston installed the wrong way.
The guy didn't mention any funny noises prior to the breakdown, just said he started the saw as he always does, let it idle a short while, picked it up and started to cut - BANG.

Some more pics: The rings look good, not worn unevenly and not rounded at the corners. The piston top looks normal but probably got banged against the cylinder ceiling a number of times after coming loose, which also might explain the shiny portions along the edges of the break in the pin bore.









I found this rather excessive casting ridge in the cylinder intake... but there are no marks indicating something might have caught there.



Difficult to see on this pic

 

[AKDoug]

See that shiny spot on the top. That's where the piston rocked in the bore and either contacted (or got really close to hitting)

 

[SEAM]

Yupp, I see it now, there is a corresponding portion in the cylinder above the intake... that might very well explain it.

Thanks!

 

[angelo c]

I suggest he not use an Impact Wrench

 

[SEAM]

I don't think the clutch was ever serviced/removed on this one

I'm still not 100% happy with the suspected cause of the "explosion". There is a lot of carbon deposit on the surface of the cracked portion. It should be almost carbon-free - with the piston disintegrating like this the crank did not do more than a few revolutions chewing up all the bits and pieces.

It probably will remain a mystery...

Thanks for the input!!

 

[Tractorsaw1]

Hey at least there's a skirt left on 1 side, I have 1 there weren't any skirt left. 10,000 rpm to zero in half a second isn't a fun sound. Yep that's gonna leave a mark

 

[crzybowhntr]

The guy you sold it to, was he expecting you to warranty this? Just curious....

 

[Chris-PA]

It's tough to determine if that mark on the top of the piston is the root cause since so much damage occurred after the thing came apart. Pretty clearly there was some rocking around afterwards too.

I was asking about that shiny part of the crack, as if I recall correctly a cyclic failure will sometimes show a polished area inside the crack. Basically the two sides of the crack polish each other as the part flexes and the crack grows. Often there will be some characteristic marks to the polished area, and if you see that it means that this was a defect that was there before things came apart.

 

[Stihlofadeal64]

this to the winner. But no one has absolutely claimed it yet.


It's Friday and there is still time

 

[lone wolf]

this to the winner. But no one has absolutely claimed it yet.


It's Friday and there is still time

I still say it slapped apart.

 

[Nitroman]

Last year I bought a 2002 Polaris 800 RMK. It was the Sno-Chek so it had the fancy graphics and clicker shocks. Since it was in Kenai I asked my BIL to go over and check it out. Guy fired it up and BIL said it "sounded good".

When it got here manager of local Polaris stealership called me to come over and check it out. Both pistons were cracked on intake and exhaust sides with the MAG side missing the intake skirt which was embedded in the head. $3000.00 and everything that could wear out was replaced.

Does not take much clearance in the cylinder bore to get things going. Again, take the jug to a machine shop and ask them to check the jug for roundness.

One thing that you can do to help prevent this is to let the saw warm up so the piston can fill the bore properly. Sometimes they just break.

 

[JDNicol]

I've got a box of five 026's that have pistons on the intake worn paper thin. So much that the pistons were rocking in the bore and contacting the squish band.

Husqvarna 61

I have seen it before where the crank counterweights bust off the exhaust side of the piston before the intake side actually fractures from wearing too thin.

 

[Grande Dog]

Howdy,
Couple of things that seem strange, and it could just be the photos. The cylinder seems to have way more carbon than the piston. The spark plug ground electrode got really hot. In the cylinder picture, there looks to be a rub towards the flywheel side of the intake port. Is there a corresponding rub on the clutch side of the exhaust port? To me it looks like the piston was pulled apart. We all know the intake side piston skirt wears a little quicker. With the amount of skirt left on the exhaust side, it would seem likely. Looks like the piston was pulled apart to me. I'm thinking the shiney marks were just from the piston pin taking a swipe at it the piston after it broke.
Regards
Gregg

 

[SEAM]

I cleaned the piston outer surface some for the photo shoot

Might have been a number of things adding up... a gradually forming crack around the pin would increase piston wear, causing slapping and finally this.

Thanks again for the ideas...

 

[jwilly]

I'd just say it blew up!
Why?
Cuz it happens.

 

[rattler362]

SEAM I don't mean to trample on your thread but here is one a guy brought me to repair he said it was idling when this happened good thing too this piston measures 2.180

 

[SEAM]

That's a nice one, too

Cutoff saws are prone to skirt wear if the filter doesn't seal exactly or oil isn't applied regularly...
What puzzled me in my case is the absence of any transfer or powdery substance and score marks.
But, as was said in this thread before, 024s tend to wear thin on the intake side. I have a number of other 024/026 saws that need an overhaul. I'll check the skirts when I work on those.

 

[rattler362]

It dose have a bit of scoring on the ext side but not a lot but the piston looks wore to me time for some clean up and check the bearings if they are tight we will go from their.

 

[drf255]

FWIW,

A seller on eBay has original 44.7mm Stihl pistons for sale for $25 shipped with rings, no pin.

I'm not sure if yours is a 44 or a 44.7.

 

[drf255]

My beauty.




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