Stihl 026 piston hitting cylinder. Paging Chainsaw King D'Animal. (1 Viewer)

[fj40charles]

I have a Stihl 026 Pro saw that was making some noise when it was running. I was able to get it make noise just by turning the clutch by hand. The noise happened when the piston it was at TDC.

Removed the piston and cylinder to find this.

Piston wrist pin and bearings are tight.

In the last pic, you can see excessive wear on the piston skirt (left side)

Any ideas Dan?

Please see attached pics.

 

[fj40charles]

pics of cylinder. You can see where the piston touched the top of the cylinder on the exhaust side.

 

[fj40charles]

more pics of cylinder.

 

[fj40charles]

pics of case

 

[fj40charles]

case. other side.

 

[e9999]

interesting diagnostic

nice to see the inside of one of those. Beefy.

put a thicker gasket in as a quick temporary fix? but yes, I'd want to know what is going on. D' to the rescue!


(I would have cleaned it a bit before opening it up, but one might want to live dangerously instead I guess... )

 

[fj40charles]

some pics of the piston. You can see from the measurement that one side is taller than the other.

Top pic is the intake side.

Bottom pic is the exhaust side which was hitting the top of the cylinder.

I know this is not a real accurate measurement, but this was what I have at home.

 

[fj40charles]

more pics..

"E" on the blue tape shows the exhaust side of the piston.

The black up arrow is where the high spot is in the piston. This part did not contact the top of the cylinder, but you can see how the piston top on the left and right side of the arrow is lower.

 

[fj40charles]

Any chance it is a manufacturing defect of the piston? Looking at the markings, it appears to be a genuine Stihl piston and not aftermarket.

 

[fj40charles]

Top of piston pic.

 

[Rambrush]

Something must be loose, How old is this saw?
You say you checked and there is no slop in the crank or rod bearings and the wrist pin was tight this is strange. The pistons don't stretch that I have ever seen. On the exhaust side there is a screen and nothing came in that direction?
Nothing that you know of dropped in the spark plug hole?
I have seen the skirts have problems and then it will allow the piston to shift move in the cylinder wondering about that. This of course would cause a situation like you describe clearance on one side and hitting the other side because its crooked in the bore.
Prob a new cylinder and piston will be the cure

 

[e9999]

you haven't told us much of the history of this thing IIRC besides the excellent pictorial description of the situation today.
Did it start making this noise progressively/abruptly under your watch or is it a bought-used-always-made-the-noise situation?
Could the gasket (yes, again, I'm on a roll with the unlikely scenarii today... ) have been changed mistakenly to a thinner aftermarket one or compressed enough to allow contact? Or did something get hot enough that it deformed?

OK, ya, it's just probably crooked...

Can't wait for Dan to set us all straight... Where is he and why is he not checking this forum 24/7? We need him all the time for Stihl Toys... Does he have a life outside of MUD or somethin' ? sheesh...

 

[Rambrush]

I would sure bet on the piston rocking notice how the marks are and where they are not. Indicates it is rocking in the direction but not the other.
A gasket would smack the top all the way across. possibly not even rotate. A rocker will run and knock like a old model T

 

[fj40charles]

I recently purchased this saw used. I heard the noise when it was running, but didn't think much of it since I've heard other saws sound like that. Only after holding the clutch and manually turning the crank, was I able to notice the sound at TDC.

I will be getting an OEM piston and cylinder kit for it.

Rambush,

You were correct. I put the piston back into the cylinder and I can see it rock back and forth. There is about 1/16" of an inch of play.
I think that explains how the top of the piston hit the cylinder.

Thanks,
Charles

 

[D'Animal]

Your piston is worn out.

Look at the #3 picture in you first post.

The bottom of the piston skits should be the same thickness. You have one side that is worn and one side that is worn paper thin. The wear allows the piston to rock or pivot in the bore. This allows the piston to touch the cylinder.

Install a new piston and you will be ready to go.

 

[fj40charles]

Your piston is worn out.

Look at the #3 picture in you first post.

The bottom of the piston skits should be the same thickness. You have one side that is worn and one side that is worn paper thin. The wear allows the piston to rock or pivot in the bore. This allows the piston to touch the cylinder.

Install a new piston and you will be ready to go.

Dan,

Thanks for the reply. I will buy a new piston for it.

Charles

 

[Rambrush]

Glad I was able to assist..

 

[dohcdelsol93]

in pics # 5 and # 6 that cylinder looks super scored as in its trashed. If you buy a new piston and put it in that cylinder you will ruin your brand new piston. Some shade trees will say sand it down so it's smooth and run with it. Without that protective cylinder coating your internals won't last long. It might be just good enough to run it for a pawn shop clerk to hear it run and get some beer money.

Do your self a favor and buy a piston and cyliner set (along with a new base gasket as it does not come with the stihl piston/cyl kit)

Dont let your local shop talk you into that cheap china made knock off aftermarket crap either. Those china made cyl/piston kits are crap...buy OEM Stihl only.

and before you purchase the kit find out WHY it's so scored.

Did you run ethanol in your fuel and the alocohol seperated from the oil...did you mix too little oil or run with straight gas, did you run it with cheap and/or unapproved mixing oil?

Or did this happen because of an air leak (leaky impluse hose/fuel line/carb issue/leaky crank seals)

Make sure the bottom end is in good shape. I've seen many saws make have cyl/piston contact because of worn crank bearings. without the cylinder installed you should have no in/out or up/down play on the crank shaft (wiggle both sides to check this)

If it does wiggel this is very common when people use the tree pawls (teeth at front of the saw that meet the tree) as dull chain over-rides. This puts excessive wear on the clutch/crank and will wear out the crankcase bearings prematurely.

In some cases it can even wear out the crankcase housing in the saw where the bearing presses into the housing(seen it in many MS 200T's) but in an older saw like an 026 it's something you want to check out before ordering non returnable parts.

 

[fj40charles]

Cylinder cleaned up real nice. Got some 240 grit aluminum oxide ball hone (1.75" for 44mm piston) to clean up the glazing on the cylinder. The pics look much worse than the actual condition of the cylinder wall. Nikisil coating pretty tough and does not wear very much. I spent about 20 seconds total honing it out on low speed of my cordless drill.

Installed a brand new Stihl OEM piston, wrist pin, clips, and cylinder gasket. Started on the second pull.

Had a chance to run it over the weekend trying to break it in. Running it about 3/4 throttle cutting wood.

Still need to burn a few more tanks of fuel before complete break in and before full power.

All bearing were in good condition without any slop.

 

[splitshot]

Congrats Charles, feels good don't it!

Funny thing is, the last one rebuilt becomes the first "to reach for" instead of the newer unit.. Can't figure that out..