What did this 576XP ingest?

[Fishnuts2]

I knowingly bought this 3 yr old 576 from original owner today with little or no compression.

Pulled the muffler and plug and saw very little scoring, but found the problem when I pulled the jug. It looks like something came in by a transfer and had a hard encounter with the piston.

If it was a crank bearing, wouldn't the crankcase be a little torn up? The pictures show how clean, and oily the crankcase is. The wrist pin and bearing are clean and full of oil too.

The throttle plate screws are intact and there was no evidence in the muffler of anything either. I know I'm the first to tear it apart because of all the saw cake on it.

What do you guys think?

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[fearofpavement]

Piston pin circlips accounted for? Bearings not missing any pieces? Tip or piece of insulator broke off a spark plug (perhaps a previously installed one)? Someone put a screwdriver in the cylinder to use as a piston stop and it broke a piece of the screwdriver tip off?
Lots of possibilities but make sure you know there's no more parts floating around before reassembly.

 

[Fishnuts2]

I forgot to mention the piston pin circlips were just fine too. The plug looks old enough to be the original. The saw was used as a back up and doesn't seem to have many hours on it.

 

[fearofpavement]

muffler screen in place?

 

[dall]

i had a piston ring line up pin come out of a cheap piston once and why i went to meteor pistons

 

[Fishnuts2]

muffler screen in place?

Yes it's in place.


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[Fishnuts2]

i had a piston ring line up pin come out of a cheap piston once and why i went to meteor pistons

Pins are in place. The damage pounded the ring lands tight enough to stick both rings.

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[ccronin]

I have seen 2 Pistons break and wreak havoc like on these saws. There is a new piston and ring set from Husqvarna. I don't know if it was heat or stress that broke them but I put new Pistons in them and they have run well ever since.

 

[fin460]

I had a similar piston, out of a super clean, low hour stihl 046. The con rod bearing let out a piece, and the piece went up through the transfer port. everything else was fine, a small nick in the transfer port. I ended up selling it to a local guy who smashed his 046 and the crank and topend were still good in his, I sold it all to him, and he got it running again.

 

[Sparkysparks9]

Definitely check that lower rod bearing well!!

 

[jughead500]

Possibly a spare chunk of flashing made its way out of the muffler and back into the exhaust port.

 

[tickhound93]

Could be any number of previously stated things, weird stuff can and does happen. Regardless what I do in that situation is replace all bearings when you rebuild. Also make sure the carb has all pieces intact. I had a polaris come into the shop that broke the jet needle off and weasled its way into the top end and simulated a loose piston circlip.

 

[DSS]

I'd say you have a bearing cage coming apart

 

[Chainsaw Jim]

Keep rotating your crankshaft until you spot the bad spot on the big end bearing cage. I'm not sure if the case bearings have a welded metal cage or not, so get a good look at those too.
Here is a pic of what you may find....



Here's a pic of the piston it claimed....



The steel the bearing cages are made from is very hard and the smallest piece will create significant damage to soft aluminum. The damaged spot on the bearing was hidden and felt like a good bearing until I rotated it enough times to expose it.

 

[wde_1978]

Keep rotating your crankshaft until you spot the bad spot on the big end bearing cage. I'm not sure if the case bearings have a welded metal cage or not, so get a good look at those too.
Here is a pic of what you may find....

View attachment 473627

Here's a pic of the piston it claimed....

View attachment 473629

The steel the bearing cages are made from us very hard and the smallest piece will create significant damage to soft aluminum. The damaged spot on the bearing was hidden and felt like a good bearing until I rotated it enough times to expose it.

Been there, the conrods needle bearing outer cage gave up on my Dolmar MS-30U weedeater, a fairly large piece made its way into the combustion chamber and eventually got chewed by the piston at full WOT!
A rebuild was financially un economic so I called it a total loss.
That was a disappointing experience!
I still miss that weedeater, had good power, extreme rpm's and great handling.

Will post up some pictures of the carnage once I get to my desktop PC.

I second the bearing failure theory as that damage looks very much like my totaled weedeaters piston.

 

[CR888]

Does the exhaust muffler can rattle when you shake it?

 

[Treespotter]

That doesn't look quite similar Chainsaw Jim. That looks exactly the same.

Wolter

 

[Fishnuts2]

Does the exhaust muffler can rattle when you shake it?

Checked that right away, looking for the piece that damaged it, and the can was empty. That's what is so puzzling, in that I can't find anything left behind.

 

[Fishnuts2]

Keep rotating your crankshaft until you spot the bad spot on the big end bearing cage. I'm not sure if the case bearings have a welded metal cage or not, so get a good look at those too.
Here is a pic of what you may find....

View attachment 473627

Here's a pic of the piston it claimed....

View attachment 473629

The steel the bearing cages are made from us very hard and the smallest piece will create significant damage to soft aluminum. The damaged spot on the bearing was hidden and felt like a good bearing until I rotated it enough times to expose it.

Jim, I think you got it. That piston damage is identical to mine. Now I have a lot of work to find and repair the damage. I'm starting part-time at a saw shop so that will be helpful.
Or maybe I should just part it out and not stick any more into it?

 

[RIDE-RED 350r]

I have seen damage like that on snowmobile and ATC engines over the years. It's almost always some sort of bottom end failure, specifically lower rod bearing or crank bearings.