Manufacturing defect or did it run lean?

[Timberwolf]

Pics from a 460, low hours, nothing showing as a problem air leak, tank vent or fuel line ect.

The saw was maybe set a bit rich and there was fairly significant carbon considdering low hours.

Anyway there was a bit of a rub on the exhaust side of the piston.

The way the rub lines up with the casting flaw I wonder if that could have been the cause. Picture does not show it, but the bevel on the port edge was not well done right were the casting plugs in the port joined.

If that is not what did it I don't know, maybe carbon.

 

[Timberwolf]

Good news is it all cleaned up leaving only a little superficial marking. Not Stihls best casting, but seen a few of them like this the last couple years.

 

[SkwerI]

It is kind of shiny on that ridge at the edge of that port. Perhaps getting those pictures to the right person at Stihl might get you a free replacement P+C.

 

[NeTree]

Looks like the lack of relief in the exhaust port let the rough edge expand into the piston. Defect for sure. Warranty?

 

[Timberwolf]

UHHHMMM, UUUUUHHHHH. Might be too late the waranty is kinda void so to speak at this point.

Marks are really superficial so won't hurt anything, I'm thinking it might just be a good idea to smooth the top of the exhaust port a little.

 

[SkwerI]

Well you're the expert, we're just trying to follow along.

 

[NeTree]

It fell apart! Honest!

 

[sotc]

Somebody maybe run 30w through it?

 

[Timberwolf]

Does not sound likely the saw was abused, aside from being a tad rich just cant come up with anything but the possibilty that the port finnish on the port edge started the rubbing.

Anyway I think it's been fixed.

 

[SkwerI]

The last time I saw a piston with that much buildup on it, the owner had been running Stihl orange bottle mix oil forever. I found my mufflers on the 200Ts were getting plugged using that oil as well. Switched to Echo (semi synthetic) and then eventually to Stihl synthetic (white bottle) and haven't had the huge carbon buildup issues in ages.

IMO the Stihl orange bottle oil stinks. Lots of better oils available.

 

[bill22]

Skwerl,Dan runs 32:1 mix in his saws still,he just pulls the screen out and goes.

 

[Magnus]

I'll have a stab at this. I like internet diagnostic's. It is like shooting in a mist or half blind! No way to get more fact or in put than provided.
On the bench you can alway's pull it down more or get more info some were...

To me there is a difference to running a tad rich and causing carbon build up.
A good grade two stroke oil will have to be mixed wrong in order to get excessive carbon build up.

My guess on this is that he used a oil that could not handle the temps correct, or way too much.
It could be he over loaded the saw, ran it too hot... But more likely oil is the culpit in addition to not so careful mix.
If he put in too much sometimes (carbon buildup), chances are he put too little other times (higher temp) and that in combination caused this.
Not consequent fuel mix.
It looks to me as the carbon build up caused this. Often this is seen by the ports as when piston surface is too hot, that is were oil get petrified.
Made a edge that snagged the port eventually.

 

[Al Smith]

I don't think the oil did it .What I base that on is having torn down many old saws and outboards that were ran using 30WT at 16 to 1 .Those were usually clean as a pin .

Usually carbon is from an extremely rich fuel mixture .I would suspect it had more to do with the damage showing up right in line with the parting line left in the casting in the area of the ex port.

No matter as it didn't look like it did much damage ,probabley then just a little manufacturing flaw .

 

[1savagehunter]

Just some more information - it is my saw. This saw was always run with accurately mixed 50:1 premium fuel using a large 60 cc syringe to measure out my Stihl oil mix. The saw was bought new in 2006 if I remember right. Has less than 40 hours on this saw. It has always been run at 12400 - 12600 until 9 months ago when my saw tech (shall I say ex - saw tech) told me that 12600 was to lean (hot) for a 460 and turned it down to 11600. I was upset with the feel of it at 11600 so after 4 hours or so I turned it back to 12600 and used it for 4 hours last month on a larger job and then sent it off to TW. I post this not to sound defensive but rather to provide information.

 

[dastevoe]

Brian, did you measure the dia. of the jug & slug ? were the tolerances OK ??

Steve

 

[Magnus]

I'l try to explain better...

There is huge issues involved in setting saws after numbers rather than performance.

The carbon is there on the piston as well as the scoring, no doubt in this, is it?
Big question is how it got there and should it be there?

Now.. You say you ran it 50-1, that would be 2%. This is not wrong. It was set to run fat and it perform unsatisfying to you. When it is set fat you get oil residue on crankcase, inside piston etc. When you re set it to run leaner without removing what ever was in engine/piston top/cylinder roof/crank case then this will be either washed out and burnt, or left. If it is left it will be a sticky surface unless saw has run hotter than the oil can handle and the oil get petrified (glossy hard=unburnt or carbonized burnt).
When It got less fuel it ran hotter. This can burn the oil left and leave carbon or glas.
Not just the accurate mix, can do this. If it ran to fat and oil is left, then burned it will do the trick too.

The type, quality, age, of oil is important. Also how saw is set and operated...

There is lots to be said about oil. It is not just to buy a bottle of a known brand and you are safe!
All oils have limits, temp ranges and perform differently in different saws regardless of rpm settings.

If there is excessive amounts of oil in a engine that develop too much heat it will turn in to solid form.
If the oil isn't there and engine develop excessive heat it will score piston differently as expansion of piston will be different and more.

There is a hole lot more involved in a saws settings than just rpm's. I've tried to explain this a number of times in various forums..
Timberwolf too, if I remember correct.

Checking rpm is a way to tell number of revolutions only. It states nothing of how the saw runs or what setting it prefer.

 

[Al Smith]

The only time I ever recall seeing the internals of a saw engine coating with black oily goo was due to it having the screen plugged tight . That was on a baffled muffler though and not a pressure can type which Stihl uses .

I know it's just speculation and conjecture but I can't see the problem as being oil related .

 

[Timberwolf]

I've seen far more carbon on saws tuned rich than on saws tuned right in the zone but run with a rich oil mixture.

Tollerances seemed fine in general but there was a slight high spot on that port edge. Also noticed a little bit of a poor port edge on the intake that also was starting to mark ever so slightly. I think I snapped a picture of it will check later.

 

[Magnus]

I have seen Gilardoni and Mahle and others look horrible out of the box, but sill nothing happens...

It seems when several things line up to a perfect situation, that is when you see result. To find out what caused this result is when it gets tricky.
It is not easy to tell what happened all the time, but some damages look the same all the time, different scoring can tell what happened.

I think carbon did this. I have seen a lot worse piston buildup than this too, without damage... I have seen this and even less cause more trouble than this.
It does not take much if it gets loose, a grain no bigger than a grain is all it takes.

 

[Magnus]

If you have a build up and it gets hot it can get hard, brittle and get loose. I think that is what happened here.

Here we see it very often as they change fuel from regular pump gas to alcylate. It flushes all crap up above piston and burn it.
The luck ones have it escape out exoust port, the not so lucky get this damage.

I Never change fuels with out cleaning hole engine first including carbs and hoses... Too high risks.

The pro we never see this with as they run on Aspen. But homeowners still use pump gas and have lots of trouble and costs from it.

 

[Al Smith]

For oils to pyrolite which is essentually to burn into petrolium coke it has to be in kind of like in an oven .Much that same as burning wood into charcoal .In side of a cylinder with the burning fuels it would seldom happen .Now deep in the crankcase if the cylinder is bypassing the rings and with an air leak it can and does happen on a badly worn out engine on occasion .That 's the black crud you find in a blown engine crankcase .

 

[Cobleskill]

Remember how the old engines used to smell when you tore them down, Al? Kind of a burnt cokey smell. I stopped at a local engine shop and he had an old VW engine tore down. The smell took me back to the 60's.

 

[Al Smith]

The worst I'd ever seen was small block Chevys that had been ran with dirty oil for a zillion miles .The intake manifolds cross overs would be plugged tight as a bulls butt at fly time with coked oil .They of course have better oil now days but you still have to change it periodically .

Fine dust that finds it's way into the crankcases of saw engines just adds to the problem .Another reason to keep a good filter on them .That said though ,it's only a guess on my part why the saw in question evidently had some carbon problems . Forensic examination over the net doesn't always tell the tale .

 

[Bermy]

See..this is what I like about the TH...a great exchange of information with nobody flaming or taking offence or getting snitty despite differing opinions, and very interesting and informative opinions they are I might add!!!
Ahh, the company of adults...

 

[MasterBlaster]

That's cuz...