Uneven Piston Wear on MS 660

[Wood Doctor]

Last month I asked a Stihl dealer to repair an MS 660 that had lost a small chunk off the top of the piston after about 500 hours of use. He replaced the piston, the rings, and the cylinder. After about 60 hours, the engine shut down again this week. This time we had a strange uneven wear problem. The intake side of the piston, cylinder, and rings is clean as a whistle. Here's a shot of the piston after I removed it:

However, the exhaust side of the piston is shot. The rings are burned out and fused to the walls, and the inside of the cylinder mirrors the piston with wear streaks everywhere:

The piston was actually loose inside the cylinder, so compression was almost zip. All other parts seem OK (bearings, crank shaft, etc). This time I'm going to rebuild the engine myself. Does anybody have a clue what caused this so that I can tell the operator after I fix it again? Maybe he can take corrective action. This seems goofy. TIA.

 

[watsonr]

Sure that's not a lean seize? Happens normally only on the exhaust side. Check for air leaks?

 

[Wood Doctor]

Sure that's not a lean seize? Happens normally only on the exhaust side. Check for air leaks?

Good point. I wonder if the cylinder gasket was not set properly after the dealer repaired it. The bolts seemed tight, but that's only part of the story. The impulse hose seems OK and so does the intake manifold tube. I also checked the decomp valve and that seemed normal. Still, it makes you wonder. Where else would the air get in, and why would it run OK for 60 hours and then stop dead rather suddenly?

I'm stumped.

 

[Adam Smith]

Good point. I wonder if the cylinder gasket was not set properly after the dealer repaired it. The bolts seemed tight, but that's only part of the story. The impulse hose seems OK and so does the intake manifold tube. I also checked the decomp valve and that seemed normal. Still, it makes you wonder. Where else would the air get in, and why would it run OK for 60 hours and then stop dead rather suddenly?

I'm stumped.

What kind of gas are you running through your saw? Ethanol in the lower octane rated gasoline is a two stroke killing recipe. You need to run a 90 -93 octane gas in them. This fuel has a lower ethanol rating. You prolly already know this but just wanted to give my pennys worth. There are guys on here that can explain this better than I can. Ethanol will do terrible things to your saws

 

[Whiskers]

Hey Wood Doctor, did you or the shop pressure and vac test the saw by any chance? It really looks like lean seizure to me. The obvious places are crankshaft seals or cylinder gasket, but they can show up in other unexpected places. Did you the guy using the saw notice a high idle or surging?

 

[mortenh]

Good point. I wonder if the cylinder gasket was not set properly after the dealer repaired it. The bolts seemed tight, but that's only part of the story. The impulse hose seems OK and so does the intake manifold tube. I also checked the decomp valve and that seemed normal. Still, it makes you wonder. Where else would the air get in, and why would it run OK for 60 hours and then stop dead rather suddenly?

Lean seizure may be caused by fuel starvation or air leak.


Possible fuel starvation causes:

• Blocked carb gauze filter
• Blocked fuel filter
• Blocked tank vent

Possible air leak causes:

• Split carb boot
• Cracked fuel line
• Cracked impulse line
• Crank seal

A pressure check and vacuum check might help identify air leaks.

 

[mdavlee]

Did they forget to mix the oil in on the last batch of mix? The way it's just that side and that bad it makes me wonder if it was straight gas or lean setting on the carb.

 

[blsnelling]

Yup, lean or lack of oil.

 

[GBD]

Looks like lean to me, lack of oil likely would have damaged intake side, too. Besides, piston crown looks to have been very hot on exhaust side.

 

[Mastermind]

Good point. I wonder if the cylinder gasket was not set properly after the dealer repaired it. The bolts seemed tight, but that's only part of the story. The impulse hose seems OK and so does the intake manifold tube. I also checked the decomp valve and that seemed normal. Still, it makes you wonder. Where else would the air get in, and why would it run OK for 60 hours and then stop dead rather suddenly?

I'm stumped.

I would wager that after that 60 hours of the saw running fine, something happened that caused a lean condition. I've not seen too many mysteries.

 

[Wood Doctor]

Bad Fuel Mix Could Be It.

This reminds me of Sherlock Holmes, so let's try to get to the bottom of it.

After this saw was fixed last year, I received no indication from the owner (let's call him Andy) of the saw that the saw ever ran badly, neither idling way too high--an obvious air leak indicator--nor anything else. He was proud to use this repaired saw. As long as Andy ran the saw, it operated perfectly. He reported this to me not just once but several times.

Last week Andy gave this saw to a worker that he hired to help him run his business. Usually, Andy controls the fuel that all saws in his fleet receive. Andy never cheats on fuel. However, this saw could have received fuel from another source while Andy was operating another saw in his fleet.

Based on what I read here, this MS 660 suddenly started running hot and the piston seized. I've been told that less than a half-tank of straight gas burning can cause it. Yes, it might have been an air leak with 40:1 gas, but in the end, heat is the true culprit and straight gas could have caused it all--ruining the cylinder, rings, and piston on the exhaust side.

Is my analysis correct? Please advise.

 

[barneyrb]

This reminds me of Sherlock Holmes, so let's try to get to the bottom of it.

After this saw was fixed last year, I received no indication from the owner (let's call him Andy) of the saw that the saw ever ran badly, neither idling way too high--an obvious air leak indicator--nor anything else. He was proud to use this repaired saw. As long as Andy ran the saw, it operated perfectly. He reported this to me not just once but several times.

Last week Andy gave this saw to a worker that he hired to help him run his business. Usually, Andy controls the fuel that all saws in his fleet receive. Andy never cheats on fuel. However, this saw could have received fuel from another source while Andy was operating another saw in his fleet.

Based on what I read here, this MS 660 suddenly started running hot and the piston seized. I've been told that less than a half-tank of straight gas burning can cause it. Yes, it might have been an air leak with 40:1 gas, but in the end, heat is the true culprit and straight gas could have caused it all--ruining the cylinder, rings, and piston on the exhaust side.

Is my analysis correct? Please advise.

It don't take nearly a half tank of straight gas, more like couple of ounces maybe less

 

[Wood Doctor]

Why not a lock up?

It don't take nearly a half tank of straight gas, more like couple of ounces maybe less.

Usually men report about a half-tank of straight gas because the straight gas was poured into a tank that still contained some mixed fuel. Eventually, the mixture of straight gas and correct fuel mix ruins the engine. When the fuel mix intake finally gets to about 100:1 or higher, it's all over because the engine runs hotter than a $2 pistol.

I think that could easily have happened here, but I'm surprised that the piston did not simply seize up and lock to the cylinder wall. This piston and its rings literally reamed out the cylinder to death until there was no compression. Then the saw stopped dead. There is now a loose fit between the cylinder and the piston/ring assembly. So, I am still a bit perplexed. :msp_confused:

 

[JoshF]

A lack of oil in the gas mix will cause scoring all the way around the piston. This is a lean fault

 

[wheelman]

There will be other clues to help figure out if this was a straight gassed saw or a lean seize.
Piston Failure Analysis This is a good read.

 

[Abflyboy]

Were any diagnostic tests performed?

Sure looks like a lean sieze to me ... :bang:

 

[Whiskers]

If the cylinder doesn't have a lot of time on it, it would be worth cleaning up with acid to see if there is any damage. The cylinder is a lot harder than the piston and sometimes even though the have a lot of transfer they clean up great.

 

[mweba]

Its fun to speculate but at this point, it doesn't matter. If you want to rebuild the saw, all tests need to be performed and faulty parts replaced. Otherwise you will be tossing 2-300 out the window.

If you need assistance before the Spring GTG, let me know.

 

[zogger]

All of the above, plus loose muffler, bad gasket there.

I had a great running rancher go on me, that was the culprit, and man it didnt take very long at all, seconds, not many, 15 maybe? Started sounding screwy, then I felt heat, then shut off. Inspection, one bolt just slap gone (as in WTH how did it get out??????) the other loose. Scored pretty bad on the exhaust side.

 

[Whiskers]

Hey wood doc, your pm's are full. If you don't have a pressure/vac setup let me know. I've got everything you'd need and could help you get it figured out.