Stihl piston examination

[brandonstclair20]

I took the muffler off of a running 064 I just purchased. I saw a scored piston so I took it apart. With some careful work with, fine sandpaper, scotchbrite and a Brillo pad I cleaned up the cylinder. Nothing catches a fingernail. I ordered a new oem piston on Monday. I know an air leak most likely ruined this piston. I closely examined the big end bearing and saw nothing wrong. The crank bearings are smooth. I did not see any debris in the case. I forgot tiny dents in the piston and tiny dents in the roof of the cylinder. What else could be going on here?
Thanks

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[traffic903]

Where are the rings? Were they in pieces? Are the pics of the cylinder after cleaning? If so, then I think you still have some more work left. Keep going, you will end up with a great saw once it is rebuilt.

 

[Fifelaker]

Look again at the crank bearings.

 

[angelo c]

It ate sumptin Wong ....FOD...(we used to call it on the runway)
At some point it injested or somethin let loose, bearing, circlip, maybe some chunk of metal bounced around the combustion cycle...

 

[earlthegoat2]

Did you pull the seals before looking at the crank bearings? Ive inspected crank bearings without pulling the seals before and the result can be quite misleading. After pulling the seals bad bearings are very apparent.

Other than that, a vac/pressure test can tell you what you need to know.

 

[brandonstclair20]

Where are the rings? Were they in pieces? Are the pics of the cylinder after cleaning? If so, then I think you still have some more work left. Keep going, you will end up with a great saw once it is rebuilt.

The pictures are after cleaning, I can see those marks but they can’t be felt. How do I completely remove them?

Thanks


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[traffic903]

The pictures are after cleaning, I can see those marks but they can’t be felt. How do I completely remove them?

Thanks


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I think you will be able to clear the majority of the cylinder wall marks with some more scotchbrite or light sanding. But that squish band needs some work before you run it. There must have been some debris in the case for there to be the damage to the squish and top of piston like that. Did it bust a ring, eat a bearing, ingest something, clip etc etc. Are you CERTAIN that the rod bearing is ok? What about a spark plug electrode breaking? Just throwing some ideas out there for you to check on.

 

[Fifelaker]

The piston crown on my MS440 looked almost exactly like that. The bearings felt fine at first, but after a bunch of turning the crank I found a spot that it would move a little bit. When I split it it was obvious they were bad.

 

[Marshy]

The pictures are after cleaning, I can see those marks but they can’t be felt. How do I completely remove them?

Thanks


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As long as there are no sharp protrusions you will be fine. Clean it up and reuse it. My guess is the saw failed previously and caused this and was rebuilt, or everything is related and a bearing cage has failed and been ingested.

Either way, I'd take a hard look at the bottom end. It'd be worth the money to pop the seals out and look at the cages of the bearings.

Don't want a RUD (Rapid Unplanned Disassembly). @angelo c, your not the only one with cool acronyms.

 

[fordf150]

I'm with everyone else, pull the seals or for that matter just rebuild the bottom end. bearings are cheap and you already need a gasket set so the only extra money is 2 crank bearings and the time to do it. $25 and an extra hour of time then you know its done right

 

[Iceboy]

I'm with everyone else, pull the seals or for that matter just rebuild the bottom end. bearings are cheap and you already need a gasket set so the only extra money is 2 crank bearings and the time to do it. $25 and an extra hour of time then you know its done right

+1

 

[jakethesnake]

I think you will be able to clear the majority of the cylinder wall marks with some more scotchbrite or light sanding. But that squish band needs some work before you run it. There must have been some debris in the case for there to be the damage to the squish and top of piston like that. Did it bust a ring, eat a bearing, ingest something, clip etc etc. Are you CERTAIN that the rod bearing is ok? What about a spark plug electrode breaking? Just throwing some ideas out there for you to check on.

not being smart. What does it hurt if the squish band is dinged up. ?
I have a 346 with a top end like that I intend to use. Hadn’t really even thought of cleaning it up

 

[brandonstclair20]

I cannot pull the seals until I remove the flywheel and clutch and the cylinder and piston are off. My plan is to install the cylinder and piston. Then remove the seals and inspect the bearings. At some point there was some kind of failure. I saw this on the bottom of the crank.

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[Lone Wolf]

I cannot pull the seals until I remove the flywheel and clutch and the cylinder and piston are off. My plan is to install the cylinder and piston. Then remove the seals and inspect the bearings. At some point there was some kind of failure. I saw this on the bottom of the crank.

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That looks like the shape of a rod bearing needle.

 

[brandonstclair20]

That looks like the shape of a rod bearing needle.

I looked at the big end bearing and all of the needles are present and the cage looks good.

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[brandonstclair20]

Maybe a old wrist pin bearing failure?


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[Lone Wolf]

Hard to say over the net just look at everything real close and double check.

 

[smokey7]

If the spots are raised up off the surface of the band you can get hot spots and cause preignition if the conditions were right. Also the high spots can cause squish issues. Other then that the dimples are a non issue. Got a few that way because i had no way to clean it up at the time.

 

[Marshy]

I cannot pull the seals until I remove the flywheel and clutch and the cylinder and piston are off. My plan is to install the cylinder and piston. Then remove the seals and inspect the bearings. At some point there was some kind of failure. I saw this on the bottom of the crank.

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Stick a hammer handle onto the bottom end and take the clutch and flywheel off. Might save you time and aggravation.

 

[RIDE-RED 350r]

Pecker tracks like that on the piston crown and squish band indicates debris from a failed bearing in the bottom end.