# Piston scoring on exhaust side



## redunshee (May 2, 2013)

Picked up an 028 Super with a scored piston and cylinder on the exhaust side. I was able to clean up the cylinder but will need to replace the piston. What would cause this scoring? I presume an air leak would cause a lean seizure on the intake side. I want to address any issue before I run saw.

Bob


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## WetBehindtheEar (May 2, 2013)

Well, I had this freind, see... he rebuilt his saw and had it running like a top...

Then he figured he'd fiddle with the H screw on the carb a little to see how many more RPM's he could get in the cut.

Turned it a little too far & leaned out the mix too much. 

The chips were really flying right before the saw locked up.

Did it look like this?:
View attachment 293483


That or the person who ran it put straight gas in it, no mix.


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## redunshee (May 2, 2013)

Cylinder was no where that bad. Cleaned up really well. So it may have been a too lean Hi speed adjustment or bad fuel/oil mix?


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## WetBehindtheEar (May 2, 2013)

redunshee said:


> Cylinder was no where that bad. Cleaned up really well. So it may have been a too lean Hi speed adjustment or bad fuel/oil mix?



My understanding is: 

Most scoring on the exhaust side is because the exhaust gasses are too hot. Lack of lubrication or high (fuel/air) to oil mix will generate enough heat to essentially melt the aluminum piston. I knew exactly what I did when it happened & I about cried. 

I actually fixed that cylinder & put a NOS piston in it and that sweet saw started right up - plus I've got 150psi compression. 

There are a ton of guys on here who will be able to describe the nuances of why a lean siezure happens.


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## bplust (May 2, 2013)

If you're gonna rebuild the saw, you have to leak test it, otherwise you're not doing a complete job. The cause of the damage could have been an air leak, bad mix, or a heavily carboned exhaust port that let a chunk go... into the cylinder. Or a barfly modding the muffler while still attached (just kidding, but ya never know...).


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## cedarshark (May 2, 2013)

I suspect an air leak either at a crank seal, impulse line,fuel line or carb boot. It could be a lean mix problem at the carb but you should do a vacumn/pressure test to eliminate or confirm the presence of a leak. Chances are most of the rubber goods on the saw should be replaced anyway due to age. You can rebuild the carb w/ a kit at the same time and reset the carb to factory settings. Lots of info on AS on all the above. 028 is a nice firewood saw when you complete the project.


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## redunshee (May 3, 2013)

I always do a pressure/vac test when rebuilding a saw. Impulse and fuel lines are good as well as intake boot. Seals we'll see. Reason I asked the question was most leaks I've seen happened on the intake side and not exhaust.


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## Fish (May 3, 2013)

Lean seizure can be caused by many things, and usually the damage starts around the exhaust, but can bounce across to the other side as well.

No oil in the mix usually shows scoring all around the piston, but usually worse closer to the exhaust for obvious reasons.

Our modern saws could be adjusted to run @ 17,000 rpms or higher, but aren't because the piston will melt, so the mixture is richened up
to slow it down to @ 13,000 or so.

An air leak does the same damage as mis adjusting the carb, as excess air is leaning out the air/fuel mix, as the air is just entering from
somewhere other than the carb. A hole in the fuel line can cause the same thing, as it is starving the carb of fuel, making it run lean in that 
way.

Running lean on fuel does a double whammy. Not only the engine runs too fast, but since the oil is in the fuel mix, you are going way too fast with way less lubrication, making the temps rise exponentially.


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## buck futter (May 3, 2013)

sometimes built up carbon can scratch the piston, its hard to know unless we can see a pic.

if you see transfer onto the cylinder it is basically not enough oil or lean f/a ratio.


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## cedarshark (May 3, 2013)

redunshee said:


> I always do a pressure/vac test when rebuilding a saw. Impulse and fuel lines are good as well as intake boot. Seals we'll see. Reason I asked the question was most leaks I've seen happened on the intake side and not exhaust.



I've only rebuilt about 30(miniscule compared to others here) and my experience is just the opposite. Most of the scoring is on the exhaust side.


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## trophyhunter (May 3, 2013)

Fish nailed it, that's one of the best explanations of lean related failures I've read on here.


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## Fish (May 3, 2013)

trophyhunter said:


> Fish nailed it, that's one of the best explanations of lean related failures I've read on here.



Well, just one of the rare times that I am sober...............


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## trophyhunter (May 3, 2013)

Fish said:


> Well, just one of the rare times that I am sober...............



It's early, I mean before noon anyways. The point is, why wait any longer? :msp_biggrin:


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## cedarshark (May 3, 2013)

Fish said:


> Well, just one of the rare times that I am sober...............



Fish..At what point(age) would you replace the rubber goods on a saw to avoid future problems with air leaks ? Sometimes, waiting til you have an air leak means a lot more work and $.


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## cmarti (May 3, 2013)

cedarshark said:


> I've only rebuilt about 30(miniscule compared to others here) and my experience is just the opposite. Most of the scoring is on the exhaust side.



Same here.


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## redunshee (May 3, 2013)

Thanks for all the input. I've closely inspected the impulse line, fuel line, and intake boot. All seem in excellent shape. Next step is to install new Piston, rebuild carb and do a pressure/vacuum test.
Bob


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## HuskStihl (May 3, 2013)

Lean conditions are going to score the exhaust side first. Leaks, no matter where the leak itself is located, will lead to a lean condition (if not recognized) and eventually to exhaust side scoring. If any of this is wrong, Fish will set me straight, exponentionally:biggrin:


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## cedarshark (May 3, 2013)

redunshee said:


> Thanks for all the input. I've closely inspected the impulse line, fuel line, and intake boot. All seem in excellent shape. Next step is to install new Piston, rebuild carb and do a pressure/vacuum test.
> Bob



Seal the intake and exhaust ports. If the saw fails the vacumn test, pressure it up to 7 or 8 psi, and completely submerge it under water in the sink. The bubbles will quickly tell you where the leak is from. This will also verify whether you did a good job sealing the intake & exhaust ports so you are not chasing a "false leak". No....I'm not pulling your leg.:cool2:


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## redunshee (May 3, 2013)

cedarshark said:


> Seal the intake and exhaust ports. If the saw fails the vacumn test, pressure it up to 7 or 8 psi, and completely submerge it under water in the sink. The bubbles will quickly tell you where the leak is from. This will also verify whether you did a good job sealing the intake & exhaust ports so you are not chasing a "false leak". No....I'm not pulling your leg.:cool2:



Yup. That's how I do it. That's how I saw a leak at the sparkplug hole as I was testing thru the impulse line on another saw.


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## 3000 FPS (May 3, 2013)

I just rebuilt an 028 super that had a been sitting for awhile. Had a good piston and cylinder in it. I replaced the seals, intake boot, fuel line, and impulse line as SOP. Not that expensive and decided not to chance it. If I do not do both seals I always do the PTO side because it just seems to wear sooner.


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## redunshee (May 3, 2013)

3000 FPS said:


> I just rebuilt an 028 super that had a been sitting for awhile. Had a good piston and cylinder in it. I replaced the seals, intake boot, fuel line, and impulse line as SOP. Not that expensive and decided not to chance it. If I do not do both seals I always do the PTO side because it just seems to wear sooner.



Yea, I've been toying with replacing those items. Did u buy oem?


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## 3000 FPS (May 3, 2013)

redunshee said:


> Yea, I've been toying with replacing those items. Did u buy oem?



The seals I bought OEM but the rest aftermarket.


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## R T (Nov 10, 2017)

I have a question, which follows along this line of discussion, that I hope someone can help answer.

I have (had) a Stihl 291 C that just scored the cylinder. I bought it exactly 5 years ago and, while I've only used it for light-duty work, last year I used it extensively to cut 8 cords of wood. I never did anything to it, other than sharpen the chain, replace the bar, and clean the outside housing. I always used hi-octane fuel (91 or higher) mixed with the Stihl 2-stroke oil. A few days ago, I lent my saw to a friend of mine, AND gave him the mixed fuel to use as well, knowing that he would use the proper fuel for the saw. He called me up and told me that it got really hot and stopped working. I took it into the shop from where I bought it and they showed me the scored cylinder wall (using a flashlight to look into the cylinder through the sparkplug hole).

My question is how did the cylinder wall get scored? Did my friend use straight fuel? Did I omit to perform annual maintenance and thereby missed something? Is this just a case of "Well, it happens; buy another saw and move on."


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## a. palmer jr. (Nov 10, 2017)

R T said:


> I have a question, which follows along this line of discussion, that I hope someone can help answer.
> 
> I have (had) a Stihl 291 C that just scored the cylinder. I bought it exactly 5 years ago and, while I've only used it for light-duty work, last year I used it extensively to cut 8 cords of wood. I never did anything to it, other than sharpen the chain, replace the bar, and clean the outside housing. I always used hi-octane fuel (91 or higher) mixed with the Stihl 2-stroke oil. A few days ago, I lent my saw to a friend of mine, AND gave him the mixed fuel to use as well, knowing that he would use the proper fuel for the saw. He called me up and told me that it got really hot and stopped working. I took it into the shop from where I bought it and they showed me the scored cylinder wall (using a flashlight to look into the cylinder through the sparkplug hole).
> 
> My question is how did the cylinder wall get scored? Did my friend use straight fuel? Did I omit to perform annual maintenance and thereby missed something? Is this just a case of "Well, it happens; buy another saw and move on."


 This is kinda hard to diagnose since we weren't there but your saw got a lean mixture somehow..You can do a pressure test on it and know if it had an air leak but it's hard to know if your friend leaned out the carb trying to get it to run faster or if he might have switched gas cans or something...


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## Conquistador3 (Nov 10, 2017)

R T said:


> I have a question, which follows along this line of discussion, that I hope someone can help answer.
> 
> I have (had) a Stihl 291 C that just scored the cylinder. I bought it exactly 5 years ago and, while I've only used it for light-duty work, last year I used it extensively to cut 8 cords of wood. I never did anything to it, other than sharpen the chain, replace the bar, and clean the outside housing. I always used hi-octane fuel (91 or higher) mixed with the Stihl 2-stroke oil. A few days ago, I lent my saw to a friend of mine, AND gave him the mixed fuel to use as well, knowing that he would use the proper fuel for the saw. He called me up and told me that it got really hot and stopped working. I took it into the shop from where I bought it and they showed me the scored cylinder wall (using a flashlight to look into the cylinder through the sparkplug hole).
> 
> My question is how did the cylinder wall get scored? Did my friend use straight fuel? Did I omit to perform annual maintenance and thereby missed something? Is this just a case of "Well, it happens; buy another saw and move on."



First possibility, he used straight gas or, to be more precise, used up most of the pre-mix you gave him and merely added gasoline to it, resulting in gas with minute traces of oil in it. 
Second possibility, he ran the saw without air filter. Just ask your Stihl dealership what otherwise sane people do with chainsaws, leafblowers and the like and then complain "it's not working anymore". 
I am not a seer, I've merely seen it happen so often it's not even funny anymore.


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## Ozhoo (Nov 10, 2017)

R T said:


> My question is how did the cylinder wall get scored? Did my friend use straight fuel? Did I omit to perform annual maintenance and thereby missed something? Is this just a case of "Well, it happens; buy another saw and move on."



The first rule of Chainsaw Club is, "NEVER loan out a saw"
The second rule is, "If you DO loan out a saw, it SHOULD be a Electric"


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## a. palmer jr. (Nov 10, 2017)

Ozhoo said:


> The first rule of Chainsaw Club is, "NEVER loan out a saw"
> The second rule is, "If you DO loan out a saw, it SHOULD be a Electric"


 That's why they invented the Wild Thing...


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## Mad Professor (Nov 10, 2017)

R T said:


> I have a question, which follows along this line of discussion, that I hope someone can help answer.
> 
> I have (had) a Stihl 291 C that just scored the cylinder. I bought it exactly 5 years ago and, while I've only used it for light-duty work, last year I used it extensively to cut 8 cords of wood. I never did anything to it, other than sharpen the chain, replace the bar, and clean the outside housing. I always used hi-octane fuel (91 or higher) mixed with the Stihl 2-stroke oil. A few days ago, I lent my saw to a friend of mine, AND gave him the mixed fuel to use as well, knowing that he would use the proper fuel for the saw. He called me up and told me that it got really hot and stopped working. I took it into the shop from where I bought it and they showed me the scored cylinder wall (using a flashlight to look into the cylinder through the sparkplug hole).
> 
> My question is how did the cylinder wall get scored? Did my friend use straight fuel? Did I omit to perform annual maintenance and thereby missed something? Is this just a case of "Well, it happens; buy another saw and move on."




As Mr Palmer said that is why to have a $25 wild thingy or $50 super homie XL ( love those XL saws but can be had cheap) if someone asks to borrow a saw.

Giving out the loaner with fuel is advised and insurance but not a cure for stupidity.

See attached for scoring analyses.


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## Deleted member 117362 (Nov 10, 2017)

R T said:


> I have a question, which follows along this line of discussion, that I hope someone can help answer.
> 
> I have (had) a Stihl 291 C that just scored the cylinder. I bought it exactly 5 years ago and, while I've only used it for light-duty work, last year I used it extensively to cut 8 cords of wood. I never did anything to it, other than sharpen the chain, replace the bar, and clean the outside housing. I always used hi-octane fuel (91 or higher) mixed with the Stihl 2-stroke oil. A few days ago, I lent my saw to a friend of mine, AND gave him the mixed fuel to use as well, knowing that he would use the proper fuel for the saw. He called me up and told me that it got really hot and stopped working. I took it into the shop from where I bought it and they showed me the scored cylinder wall (using a flashlight to look into the cylinder through the sparkplug hole).
> 
> My question is how did the cylinder wall get scored? Did my friend use straight fuel? Did I omit to perform annual maintenance and thereby missed something? Is this just a case of "Well, it happens; buy another saw and move on."


Is the piston scored on intake and exhaust sides?


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## 3000 FPS (Nov 13, 2017)

That is a good question. Is the cylinder scored on both sides. If yes then I would say it was straight gassed.


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