026 piston diagnosis

[Timberwolf]

Best I know here is the deal, saw has mild muffler mod and tuning seamed ok when I ran the saw. I think it had been used mostly for limbing work in softwood.

Piston intake skirt is worn, but rings are A1 shape, end gap is about .006 in. Squish 0.024 in.
Exhaust side of the piston crown and head area above the exhaust totally eaten up.
Some of the errosion was recent, some was quite old as it was carboned over.
Outside edge of piston crown above top ring was squeezed out to the bore and worn so there was alot of heat in the crown.

I am thinking about recutting the head surface and using a new piston, but not sure how deep the head is dammage and if cutting 0.020 or so will get it back to decent metal.

 

[SkwerI]

Detonation from low octane fuel? The 0.024 squish isn't the stock setting is it? I'm assuming the squish was reduced at some point.

 

[Cut4fun]

Intake skirt was wore allowing a little bit of tilt and being run on cheap octane and maybe a little lean for extended cuts?

 

[sotc]

50:1 gas:ball bearing mix?

 

[Timberwolf]

Intake skirt was wore allowing a little bit of tilt and being run on cheap octane and maybe a little lean for extended cuts?

Thats about what I was thinking, Some of the carbon deposit was really hard, not sure if it could have played a roll too.

The errosion in the head is not as bad as I thought, I think I should be able to cut a clean squish and lower the cylinder, but going to let the owner make that call.

Not sure about mix oil used.

 

[Timberwolf]

Brian, squish on the little stihls is tight, often they are under 0.025 factory.

 

[Magnus]

This I have seen on old saws many times.
Usually you find answer in muffler on the 242's, but on this I am not sure if they get trapped.. It could be worth a look..

It is not so bad damage so I guess one or two of these little super hard carbon pellets caused this.
The only way out is thru exoust port. As proven on piston it can take the pressure.

What oil is used in this? It has more carbon on exaust side so I believe the oil turned in to carbon instead of doing it job.
A little too hot for this oil it seems.

Hard, black, a little glossy carbon needs oil that get hot really fast...

 

[Magnus]

There is more to compression than just distance from piston to cylinder roof. It is the shape of the cylinder and piston top that decide how far from top piston needs to be right?
Bigger dome allow the piston to be as close as possible and the shape of forces as the expand to be more directly down.
Some saws allow piston all way up, but have dome so compression is not higher than in a flat that let piston turn earlier as long as the volume at tdc is the same and same amount of fuel/air mix is compressed.
I hope that makes sense...
I kan write it better in Swedish if that helps..

 

[SkwerI]

The worst oil deposits I've seen in pro grade saws have been cases where the owner used Stihl orange bottle oil. Some generic hardware store oils may be worse, but almost anything else is better IMO. The carbon buildup in my saws has been reduced by 75% or more since switching to synthetic mix oil.

 

[Stihl Crazy]

Seen this in a few 246's. One of mine with over 2000 tanks thru it had the piston fully circled with similar pits when I went in to put a new ring in it. High rpm and long cuts will do it. My 246 got a stead diet of 16,500 rpm. Okay for small logs/pulp, but those 2 foot white spruce would put some heat in them. Was using the orange bottled Stihl at the time also.

 

[CurSedVoyce]

I was thinking like Magnus on this one.. Check and see what you might find in the muffler..
I would also check and see if you might find a spring clip missing on the wrist pin or a bearing could be failing. Broken bearing cage could cause that also. Carbon is a good suspect and can burn hot and nasty.. Bad fuel, cheap mix (Napa comes to mind ), timing can contribute. Pretty much the usual suspects eh? I see A LOT of very fine dust on and around the intake boot. Leaky boot = lean with ingestion? Pitted only on exhaust side also ..... Piece of metal would probably be more all over the place... Rings look rough there too..

 

[Timberwolf]

These digs in the piston weren't physical dammage, seen what loose bit do many times and marks go all the way around not just above the exhaust, lower end clips. bearing cages ect are fine.

Does sound like the saw was fed a diet of Stihl oil, confirmed there are lots of hours on it, 6-7 years of pretty regular use.

Going to give it a shot cutting a new squish band. PITA of a thing to set up on the lathe though. Likely set it to the side to finnish a couple other projects.

 

[Cut4fun]

TW know there is a guy down this way that sales new oem stihl 44mm and 44.7mm < not sure on that one .7) top end kits for $89 + shipping. Just in case.

 

[Al Smith]

By the looks of that piston and matching area of the head deck it looks like the piston had a hitch hiker in the form of something that hammered the surfaces . It might have been the missing portion of the top ring that could have been blown to tiny bits then ejected at some point in time after it beat the pizz out of the piston .

 

[Timberwolf]

No, nothing wrong with rings. I was surprised just how tight they were end gap wise considdering the hours on the saw. Given some of the poc marks were older and some rather new and also that the marks are uniform craters with no sharp edges mechanical dammage from something does not add up. Seen lots of saws with mechanical dammage from a loose object in the combustion chamber, every one had the whole piston crown peppered and the marks had sharp edges or imprints from the shape of the pieces that were floating about. There is not a single mark on the head or piston except over the exhaust port.

Best I can figure it is like cut4fun said, too tight a squish from a piston that was worn enough to allow it to rock over in the bore. Then soon as any spot starts it makes a hot spot that just seeds more errosion around it. Alto the edge of the piston crown being pollished indicates there was high heat on the crown, that marking was also localized near the exhaust port.

 

[Al Smith]

Well I'll be the first to admit that photos over the net don't tell the whole tale. However the bassis for my comment is that at least from the perspective in the first photo it appears some of the top ring, right of the pin is missing a portion of itself .

There also is a little mark that appears on the top too .Kind of like a needle bearing hit although I'm certain it's not that else that piston would be in little tiny pieces .All this though could be a optical illusion .

I agree though cutting out the top of the head deck would be a major pain the butt .It's kind of hard to see what you're doing that deep in a hole .Plus chucking the damned thing to begin with .

One of those deals where it would take 4-5 hours to make the fixture for a 10 minute cut .

 

[Stihl Crazy]

TW, I have new and used 260 piston/cyl kits if you need any. Can put one in with your 260 and send it up.

 

[Magnus]

No, nothing wrong with rings. I was surprised just how tight they were end gap wise considdering the hours on the saw. Given some of the poc marks were older and some rather new and also that the marks are uniform craters with no sharp edges mechanical dammage from something does not add up. Seen lots of saws with mechanical dammage from a loose object in the combustion chamber, every one had the whole piston crown peppered and the marks had sharp edges or imprints from the shape of the pieces that were floating about. There is not a single mark on the head or piston except over the exhaust port.

If you run the saw leaned forward the carbon balls will bounce and fall there.
If it stands and idle after it has been hot, you get these marks all over around piston side.
Carbon balls are very hard and often sort of round, but some times oval shaped...

It is likely more than one that caused this, with some time apart.

If you idle after a couple hard cuts, airflow around cylinder isn't enough, it will at best keep high temp, but often get hotter the first 10 sec or so..
Flipping the throttle without load allow flow to increase and air to cool faster.

 

[Magnus]

Place the piston in the bore, check how much it can move. I doubt it can move enough to hit sealing. They look very different then...

 

[Timberwolf]

there is over 0.020 play between piston skirt and bore so the piston can rock over quite a bit.

Thanks for the offer on the cyl Art, I'm going to give it a shot to save the jug, if not might be hunting around for a cylinder for it.

 

[Magnus]

If it is play enough to tilt the piston the skirts have ti come of for it to tilt enough to raise one side and lower one side 0,5mm.
If you do this the rigs will still stabilize it until they are worn or groove is worn enough so it can wiggle.
Once you get it to wiggle it can hit top with one side so you get damages, but that is flat compression damages, not point pressure marks.
Piston pin bearing must be really crappy or gone. then you get marks on piston too.
The sound is something you'll never forget and feel as you let go of throttle...
Feels like it is about to explode!

 

[Al Smith]

20 thou slop is loose as a goose .Seems to me if I recall correctly that Mac kart racing engines were set up 7.5 thou with the max being 5 thou over that .Those were big old pistons though in compassison to an 026 .

 

[Magnus]

Put piston in cylinder. Have one side level with bottom and other side to the wall, measure difference to bottom. Do it again when shifting to other side.
It is a cylinder so it would be same in hole cylinder if it has same width in top and bottom. They usually do...
But not all are round....

 

[Al Smith]

But not all are round....

To get technical ,none are truely round .The face down slightly below the bottom ring will be round but from that area to the bottom of the skirt will have a slight ovality in it by design .In good not worn out condition the very bottom of the skirt would normally be the widest portion of the piston .

There are some varitaions to that design such as taper or perhaps "barrel " pistons .These however are used in automotive designs and not that I'm aware of on small engines .

 

[Timberwolf]

I think we have had this discussion before Al, I Just have not yet seen an oval saw piston, most look like they are turned on a conventional lathe for the outside finish and not a multi axis CNC. They all taper from a little below the rings to the crown but thats all I have noted.