Stihl - FS-80 Trimmer Autopsy?

[SteveSr]

Hello,

I know this is NOT a chainsaw but the engine is still a 2-stroke!

I recently inherited from a local trail club a Stihl FS-80 that died a premature death. The end cause of death was low compression but I told them that I would pull it apart to see if I could determine the circumstances leading to the low compression. Unfortunately, I never used the trimmer and have no knowledge of how it was used other than it did have a triangular metal blade on when I received it.

I pulled the cylinder off and found the following:

- The piston on the exhaust port side is severely scored.
- The cylinder on the exhaust side shows scoring and bits of aluminum piston embedded in the cylinder wall.

- The piston on the intake side looks to be in good shape. Some of the original machining marks were still visible.
- The cylinder on the intake side looks good.

- The piston skirt on the exhaust side was worn thin (much thinner than the intake side) and a section of the piston skirt about 3/16" x 1/2 " actually broke off.
- Both piston rings were worn asymmetrically towards the exhaust port side of the engine. In one place they were only about .015"-.020" thick!
- The crankcase showed signs of lubrication, a gray oily substance. I am assuming that this is mix oil and aluminum dust from the piston disintegration.
- The wrist pin and roller bearing appear to be in good shape. There is a small amount of discoloration on the pin but I can't feel any wear.

Does this provide enough information to further a cause of death? Are these symptoms typical of a particular failure mode? Any ideas on how long it took all of this damage to occur? I am wondering if the user ignored warning signs. My only thoughts are that it wasn't lubrication failure due to the oil found in the crankcase.

Your thoughts?


Thanks,

Steve

 

[fearofpavement]

Well, exhaust side scoring is often from running too lean. Although based on the rest of your data, I would say they just plumb wore the thing out.

I have an FS80 and like it a lot. It has been completely reliable for the 14 years I've owned it.

 

[eiklj]

I really like that family of trimmers. (see my sig). I'd see if you could fine a good, used engine on the bay or CL because they are worth rebuilding IMO.
FS-75, 80, & 85 used the same basic engine, only the carbs where different, mostly.

 

[jropo]

An 80 w/ a grass blade?
Sounds like it might of been over worked and under paid.

I've only seen blades on bigger units.

 

[RAMROD48]

An 80 w/ a grass blade?
Sounds like it might of been over worked and under paid.

I've only seen blades on bigger units.

I have one that has 1000's of hours on it running a Hedge Trimmer Attachment, brush blades, line heads....

Starts and runs like new...

I wouldnt put a blade on the 80 variety as it has a spring cable drive shaft but an 85 and the like are commercial duty trimmers...80 and 85 share the same engine...

 

[eiklj]

If you have the solid drive shaft like the 85 then yes, you can spin a 3 blade, 4 blade, or saw blade with ease.

 

[RAMROD48]

If you have the solid drive shaft like the 85 then yes, you can spin a 3 blade, 4 blade, or saw blade with ease.

his comment had to do with the engine, not the shaft....

 

[eiklj]

Well, I know but I just meant that the engine can handle the grass blade fine. I think the weak link was the flex shaft. At least that's what I was trying to say. Thanks. :tongue2:

 

[fearofpavement]

I use a grass blade on my FS80 with no issues. Just don't try to cut down trees with it...

 

[SteveSr]

Thanks for everyone's input. I suspect that overwork may have been part of the puzzle. This trimmer was used to cut grass and brush on the sides of hiking trails.

I think that lean mixture/overheating may have also been part. I did find the high speed screw out two turns which is twice the normal setting so I am beginning to wonder if something else was going on... like carb failure or stoppage?

As far as rebuild goes I haven't decided yet. I am assuming that the bottom end is in at least questionable shape after ingesting piston fragments and dust and ring wear detritus. It would be an easier decision if it had the solid drive shaft.

Here is an interesting scenario. What would happen if this trimmer was run hard on old fuel? I am wondering if this would be enough to cause overheating and piston seizure considering that these are mostly fuel cooled engines... Or if old fuel wouldn't produce enough horsepower for the engine to hurt itself?

Any comments?


Thanks,

Steve

 

[ZeroJunk]

I can see it having some aluminum transfer on the cylinder. But, fragments make me wonder if it didn't injest something.

Are you sure that nobody put some raw gas in by mistake ?

 

[SteveSr]

I can see it having some aluminum transfer on the cylinder. But, fragments make me wonder if it didn't ingest something.

The air filter was attached and functioning. Also there was NO scoring on the intake side of the piston.

Are you sure that nobody put some raw gas in by mistake ?

There was enough oil inside the crankcase to make me think it was not run on straight gas.

 

[ZeroJunk]

I just bought an OEM piston on eBay for $59 including shipping. $46 shipped was the best I saw on aftermarket and I figured for $13 difference I would just go OEM.

So, if you decide it's in good enough shape to rebuild that is what you are looking at.

If it was worn to the point piston slop broke a piece of skirt off chances are the rest of it is about worn out as well.