Stihl piston "hammer dance"

[Pioneer]

Tore apart a couple of trashed Stihls, the piston of an ms230 on the left, 250 on the right.

Both saws had main bearing failures, and had ingested pieces of the bearing cage. Both ran in this condition and hammered the pieces into the top of the cylinder.
Of course both pairs of pistons/cylinders are junk, which is a shame as the scoring on the cylinder walls is minimal.

 

[Yotaismygame]

So you're saying both the cylinders are beat up on the squish band is all?

 

[pioneerguy600]

If its only squish band dimpling I just sand them flat and run them, if there is cylinder wall damage then that would subjective to how deep and location.

 

[Yotaismygame]

If its only squish band dimpling I just sand them flat and run them, if there is cylinder wall damage then that would subjective to how deep and location.

That's what I was getting at too. Years ago I had a piston blow up in a 22re 4cyl engine. The piston played hammer inside the cylinder head. Machine shop I took it too told me it was just cosmetic damage.

 

[pioneerguy600]

I remove the protruding sharp edges so they don`t heat up and cause pre ignition. Other than that those dimples will fill in with carbon but won`t cause any more problems than a non dimpled head.

 

[alexcagle]

No offense, but I tend to disagree that the bearing shrapnel caused that piston pitting. I think you may have it backwards
To me, that pitting, or "BB dents", looks like damage from low octane fuel. I've seen it before, but not often.
Most people don't know it, but low octane fuel is more explosive than high octane fuel.
It can cause pre-ignition, or detonation damage, depending on the heat and compression.
It usually hammers the lower rod bearings and main bearings as well. Sometimes to failure of the bearings. Usually the lower rod bearing cages crack.

 

[Pioneer]

Both cylinders had gouges on the tapered step that allows installing the rings without a compressor. The 230 had bearing pieces in the cylinder, the 250 had pieces in the muffler. The 250 actually had 1/3 of the balls missing from the bearings and could not be found. The 230 had several missing from one bearing. It was pretty obvious that chunks past through the 250 at least, so no, I can't see how it's low octane pitting, the lowest octane around here is 87. Can't imagine what a person could run to get the octane low enough to do this kind of damage.

However, I won't deny the possibility, who knows what people wind up putting in a saw, both tanks might have been topped up with something heinous by mistake. I thought that straight gassing caused the damage, but detonation would explain all of the bearing failures.


Both pistons were badly beaten up on the bottom of the skirt on the intake side from the pieces of bearing, so they are toast no matter how you look at it. As for the cylinders, I'll put them aside just in case, but for now both saws will get new p/c kits, bearings, etc.

 

[stihl86]

No offense, but I tend to disagree that the bearing shrapnel caused that piston pitting. I think you may have it backwards
To me, that pitting, or "BB dents", looks like damage from low octane fuel. I've seen it before, but not often.
Most people don't know it, but low octane fuel is more explosive than high octane fuel.
It can cause pre-ignition, or detonation damage, depending on the heat and compression.
It usually hammers the lower rod bearings and main bearings as well. Sometimes to failure of the bearings. Usually the lower rod bearing cages crack.

You must have missed the "OK to run 87 in your saw" thread. LOL
I agree with the detonation theory. The rest of the damage came after.

 

[alexcagle]

Gas mixed with 5% diesel in your saw (have seen it happen) does some damage too. Lol..... I digress.

 

[blsnelling]

No offense, but I tend to disagree that the bearing shrapnel caused that piston pitting. I think you may have it backwards
To me, that pitting, or "BB dents", looks like damage from low octane fuel. I've seen it before, but not often.
Most people don't know it, but low octane fuel is more explosive than high octane fuel.
It can cause pre-ignition, or detonation damage, depending on the heat and compression.
It usually hammers the lower rod bearings and main bearings as well. Sometimes to failure of the bearings. Usually the lower rod bearing cages crack.

Exactly. Bad gas caused detonation which beat up the bearings.

 

[anlrolfe]

I had that happen on an old Peugeot 103 Moped Many-Many-Many-Many years ago.
It was old back then and when I rebuilt it crank rod had far too much end play AND water damage.
New Reeds and over jetted the Carb w/ new muffler.
I was running WAY OVER RPM pushing 45mph on this little POS
You'd hear the clatter but I didn't know what it was, just kept running. Back then parts were cheep and plentiful.
When it stopped it bound up 100% with a chunk of the cage in the top of the cylinder.
All the while it had been chewing small pieces.
That's OK, I rebuilt it from 50cc to 70cc.
Ever see an old school pedal style moped do 50mph?
The BIG Problem was brakes and suspension were only designed for 25
Now on to Chainsaws... For the most part I keep them OEM but have learned much here about repair and Modifications