Stihl 460 piston failure question.

[HTTR]

Anybody have an idea on what could've caused this Stihl 460 OEM piston to fail the way it did?

 

[Jacob J.]

Most of the time, when I've seen that, the saw had been run with a dirty/clogged air filter for a time, or it had been forced to run with a dull chain.

 

[HTTR]

Most of the time, when I've seen that, the saw had been run with a dirty/clogged air filter for a time, or it had been forced to run with a dull chain.

Makes sense.

 

[Tree Feller]

Most of the time, when I've seen that, the saw had been run with a dirty/clogged air filter for a time, or it had been forced to run with a dull chain.

So basically it was run hot?

 

[blsnelling]

How do you lose a section of the ring land between the two rings, yet have no damage to the rings? That seems odd to me. Was the cylinder damaged?

 

[HTTR]

How do you lose a section of the ring land between the two rings, yet have no damage to the rings? That seems odd to me. Was the cylinder damaged?

Yes it seems very odd to me as well. I've never seen anything like it. There was no cylinder damage and no additional damage to the piston.

 

[John R]

Never had a chainsaw do that, but I did have a go cart years ago with a Mc Culloch that the piston broke just like that, and as luck would have it, the piece blew out the exhaust port and lodged on the muffler, all without any cylinder wall damage.

 

[HTTR]

Never had a chainsaw do that, but I did have a go cart years ago with a Mc Culloch that the piston broke just like that, and as luck would have it, the piece blew out the exhaust port and lodged on the muffler, all without any cylinder wall damage.

Yes same here. The piston pieces were shot out into exhaust. That's what saved the cylinder.

 

[Jacob J.]

I've seen that happen on many landing saws. Heat seems to be a big factor, as does lack of lubrication between the rings.

 

[jockeydeuce]

I got a 460 as well awhile back with a matching piston........

 

[HTTR]

I got a 460 as well awhile back with a matching piston........

Yup it looks identical.

 

[StihlFroling]

Detonation causes ring lands to chunk out. Doesn't take much running pump gas. I can't say for sure that's why yours did but I have done it many times on the dyno with two strokes. And that's exactly what it looks like when you catch it quick. Can you get us a pic of the electrode strap. That should show the heat range you were operating at. Ethanol can absorb water fast when taken from a cold to hot or vise versa, especially with half empty tank or fuel can. Pretty odd that there is a matching piston as well. Check build dates?...

 

[HTTR]

Detonation causes ring lands to chunk out. Doesn't take much running pump gas. I can't say for sure that's why yours did but I have done it many times on the dyno with two strokes. And that's exactly what it looks like when you catch it quick. Can you get us a pic of the electrode strap. That should show the heat range you were operating at. Ethanol can absorb water fast when taken from a cold to hot or vise versa, especially with half empty tank or fuel can. Pretty odd that there is a matching piston as well. Check build dates?...

Electrode strap?

 

[lefturnfreek]

He's asking for a picture of the spark plug.

SBYL on the saw guy, a few of mine had a bad year also.

 

[nmurph]

Detonation causes ring lands to chunk out. Doesn't take much running pump gas.?...

Detonation will show as pock marks on the top of the piston. There's plenty of octane in 89 to prevent detonation even on saws with significantly raised compression.

Ethanol can absorb water fast when taken from a cold to hot or vise versa, especially with half empty tank or fuel can. Pretty odd that there is a matching piston as well. Check build dates?...

Gasoline can only absorb a limited amount of water from the atmosphere.

It looks like the piston in the last pic was run for sometime after it chunked the ring land.

 

[StihlFroling]

I'm just throwing out known possibilities for a ring land to chunk out, not an exact cause for sure. Hard to do from one pic. But you can crush a ring land with no other damage from deto. It would only take a drop of water to kill the octane. Like maybe when it was filled from a gas can. How could you tell how long it was run after the chunk? Is there more than just that one pic of the smooth sided piston? My apologies if I missed something.

 

[nmurph]

...How could you tell how long it was run after the chunk? Is there more than just that one pic of the smooth sided piston? My apologies if I missed something.

I'm looking at the photo of the second piston and how much oil is deposited where the land is missing. The first piston looks to have been stopped when the chunk blew.