Stihl MS 290 Engine Failure

[Wood Doctor]

This one is interesting, so I decided to post it for feedback. A fellow brought in a Stihl MS 290 that had been running fine but he said it stopped and could not be restarted. Seemed like no compression, so I decided to tear it down and rebuild it. I measured compression at 70 psi, so the top end was a gone goose. First find was a ruptured intake manifold boot. The second find was a missing piston snap ring that had come loose and wrecked the piston and rings. Here's the proof:
I inserted the scrap paper to better show the two tears in the intake boot. What I would like to know is which one failed first? The intake boot or the piston's snap ring that came loose and bounced around inside the combustion chamber until it tore up the piston, the rings, and damaged the cylinder?

BTW, the spark plug was dry as a bone as if it had been heated with a torch. And, the piston, rings, and cylinder are undamaged and clean on the exhaust side. WDYT?

 

[hotshot]

I think they cut the fastest when boots are torn, “runs great” right before they grenade.

Pure speculation, but it probably overevved & spit the clip due to the split boot causing the lean conditions....plus the piston looks dry, void of oil.

 

[Wood Doctor]

I think they cut the fastest when boots are torn, “runs great” right before they grenade.

Pure speculation, but it probably overevved & spit the clip due to the split boot causing the lean conditions....plus the piston looks dry, void of oil.

That was my sentiments also. This is the first time I have run into both occurring at once. Usually the snap ring is thrown and that wrecks the engine, but I've never had the ruptured boot show up when that occurs. The ruptured boot may have created the lean heat that eventually threw the snap ring. Surprisingly, there was no damage at all on the exhaust side. Usually that's where most piston and rings fail.

 

[dmb2613]

I don't think the lean mixture caused the problem , I would like to see more pics of the piston. Is it OEM or aftermarket?

 

[meathog]

I would vote for "aftermarket"... As well as the boot......

 

[Wood Doctor]

Boot failure would likely not be caused by the snap ring failing. Right? It seems to me that an old OEM boot that ruptured like this would cause the engine to run hot and eventually lead to piston failure one way or the other. However, why is the exhaust side of the piston and rings unscathed? Perhaps because the snap ring did all its dirty work on the intake side first -- a 50:50 chance. WDYT?

 

[grizz55chev]

Boot failure would likely not be caused by the snap ring failing. Right? It seems to me that an old OEM boot that ruptured like this would cause the engine to run hot and eventually lead to piston failure one way or the other. However, why is the exhaust side of the piston and rings unscathed? Perhaps because the snap ring did all its dirty work on the intake side first -- a 50:50 chance. WDYT?

As someone else asked, OEM or aftermarket? OEM boots are better made and last longer, after market might make a year of service, my guess is aftermarket and the snap ring let loose and did the deed.

 

[Brushwacker]

This one is interesting, so I decided to post it for feedback. A fellow brought in a Stihl MS 290 that had been running fine but he said it stopped and could not be restarted. Seemed like no compression, so I decided to tear it down and rebuild it. I measured compression at 70 psi, so the top end was a gone goose. First find was a ruptured intake manifold boot. The second find was a missing piston snap ring that had come loose and wrecked the piston and rings. Here's the proof:
View attachment 756619 I inserted the scrap paper to better show the two tears in the intake boot. What I would like to know is which one failed first? The intake boot or the piston's snap ring that came loose and bounced around inside the combustion chamber until it tore up the piston, the rings, and damaged the cylinder?

BTW, the spark plug was dry as a bone as if it had been heated with a torch. And, the piston, rings, and cylinder are undamaged and clean on the exhaust side. WDYT?

Looks to me like the boot was torn from fighting the saw out of a pinch, creating the extreme hot, lean condition that your spark plug showed which may of caused the clips to heat up , lose temper and fail, then the rest of the story... I would speculate he was in the trunk of a tree or the log, got pinched, boot unknowingly was torn prying the saw out of the pinch and very soon after enough of the torch effect from running with the lean condition softened the clips...

 

[HarleyT]

I don't think the pig was implying that the boot had anything to do with the cheapo piston clip/ring letting loose, but he was likely implying that the boot was a cheap aftermarket one....

And the common denominator in a lot of failures is "cheap aftermarket"junk....

 

[HarleyT]

The ms290 boots are pretty tough, and I rarely see a torn one. When I see one, it usually means a lot of running with bad av mounts/etc....

 

[lone wolf]

I don't think the pig was implying that the boot had anything to do with the cheapo piston clip/ring letting loose, but he was likely implying that the boot was a cheap aftermarket one....

And the common denominator in a lot of failures is "cheap aftermarket"junk....

 

[HarleyT]

Yes, Chinese is just assumed by all.......

 

[grizz55chev]

View attachment 756946

You crack me up, Wolf!

 

[lone wolf]

You crack me up, Wolf!

Thanks man.

 

[Wood Doctor]

Looks to me like the boot was torn from fighting the saw out of a pinch, creating the extreme hot, lean condition that your spark plug showed which may of caused the clips to heat up , lose temper and fail, then the rest of the story... I would speculate he was in the trunk of a tree or the log, got pinched, boot unknowingly was torn prying the saw out of the pinch and very soon after enough of the torch effect from running with the lean condition softened the clips...

So, you tend to agree with me. The intake boot ruptured first and the excess heat lead to the snap ring failure. I have witnessed boot failure before that led to piston/ring failure, but that was always on the exhaust side and never did the the snap ring fail at almost the same time. When a snap ring fails, it's usually curtains without input boot failure. Once one of those rings gives up, the top end of the engine is history.

Everything you said in your post makes sense to me. Two thumbs up.

 

[grizz55chev]

So, you tend to agree with me. The intake boot ruptured first and the excess heat lead to the snap ring failure. I have witnessed boot failure before that led to piston/ring failure, but that was always on the exhaust side and never did the the snap ring fail at almost the same time. When a snap ring fails, it's usually curtains without input boot failure. Once one of those rings gives up, the top end of the engine is history.

Ain’t much good for the lower end either.

 

[dmb2613]

Was it one of the aftermarket clips. I don't think the boot had anything to do with it

 

[grizz55chev]

Was it one of the aftermarket clips. I don't think the boot had anything to do with it

No one knows.

 

[dmb2613]

Well I thought it should be one on the other side, Looks like it is hard to get an answer for some reason

 

[TheTone]

Autopsy Report, Stihl 290:

Primary cause of death: Piston wrist pin clip failure, leading to piston scoring, loss of compression and death.
Secondary cause of death: Unknown
Possibly: (1) Intake boot rupture secondary to excessive flexion and extension due to pinched bar
(a) Inferior aftermarket boot
(b) OEM boot, defective or at end of useful life,
followed by:
(i) Excessive heat due to lean conditions secondary to (1) causing wrist pin clip failure
or,
(2) Primary wrist pin clip failure
(a) Inferior aftermarket wrist pin clip
(b) OEM wrist pin clip, defective or at end of useful life
(c) Either (a) or (b), incorrectly installed
R. Quincy, M.D.