What happened here?

[CorralLookin]

Hey guys, I inherited this 372 X-torq with a broken piston. A chunk of the skirt on the exhaust side was broken off and there were metal shaving in the exhaust. The cylinder was badly scored.

I went ahead and replaced the piston and cylinder with OEM parts. I broke in the new top end properly, and have been running it a lot ever since (1 year ago). It had been running great with no complaints.

Today, it died on me. When I went to restart it, the motor was locked up. I pulled it apart and this is what I found:

Plug looks a little lean, but not terrible IMO.


Piston is all chewed up around the top edge. Intake and exhaust side are scored. I took out the the rings. The bottom one came out easily, but the top one was partially stuck on the exhaust side. I don’t think due to excess carbon… it was just kinda pinched in there.



Big gouge in the cylinder and metal shavings fused into the top.



I’m guessing it overheated… but why is the top edge of the piston all chipped up? Never seen that before. I’d appreciate any insights!

Thanks,
Chris

 

[HarleyT]

No, the exhaust side isn't lean seized. Looks like debri/dirt ingestion, or something internal broke up.

 

[Del_]

Has that piston been hitting the head?

Sure looks like it.

Bad rod bearing or elongated rod?

 

[pioneerguy600]

Big end rod bearing broke bits off/needle bearing bits have gone up through to the top of the piston. Check the big end rod bearing on the crank throw.

 

[CorralLookin]

Yup. The lower rod bearing is busted.

 

[cscltd]

Also check the cages of the crank end bearings. You may find they may have broke up

 

[bigbadbob]

Big end rod bearing broke bits off/needle bearing bits have gone up through to the top of the piston. Check the big end rod bearing on the crank throw.

Yup got one here to.

 

[CorralLookin]

Any ideas why that lower rod bearing would fail?

 

[Bob Hedgecutter]

Any ideas why that lower rod bearing would fail?

Yep, it had reached the end of its service life.

You mentioned the inherited saw had worn the piston enough to break a chunk off the piston skirt- takes a lot of hours to get to that point- those same hours were on the conrod bearings- when they wear out fully and let go- this happens and new top ends get pretty old real quick.

 

[HarleyT]

When "rebuilding" a saw, one must seriously look at the bearings. Which is why I recommend to anyone to totally take down a saw and inspect everything before ordering any parts, and diagnose the cause of its failure the first time.

 

[Lightning Performance]

When "rebuilding" a saw, one must seriously look at the bearings. Which is why I recommend to anyone to totally take down a saw and inspect everything before ordering any parts, and diagnose the cause of its failure the first time.

Mostly avoid high hours saws myself. If you sell them to light users rebuilt they tend to survive much longer then commercially run environments. Better yet avoiding damaged anything is best for longer service life if you rebuild them.

 

[Husky77]

Check your bearings, I would say it all points that way.

 

[lone wolf]

Any ideas why that lower rod bearing would fail?

An idea here. Do you run 50-1 or worse and is the saw tuned to factory spec, and maybe it was over revved? What oil and ratio are you running, and has a tach ever checked your RPM to the stock setting?

 

[pro94lt]

Has that piston been hitting the head?

Sure looks like it.

Bad rod bearing or elongated rod?

This