Shiny piston= worn piston. Should have lines running across piston.
Stihl 026 query - purchased 2nd hand
- Thread starter macmurdo
- Start date
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doesn't look good, but kinda blurry.
That's not really wear. The tooling marks on the left are pristine.
Either too lean or straight gas in that saw's history.
Either too lean or straight gas in that saw's history.
you need to find out what damaged the piston and fix that. if it was straight gas, that's easy. but it could be an air leak. also, the cylinder may well be damaged also. first thing to do is a vacuum and pressure test. there are various threads on here detailing how to do that. you will need a mitey vac or something comparable. you need to do both tests. after that's done, remove the cylinder and get a look at the damage. you can also check the crank and rod bearings for wear or damage at that time.
in the photo, it looks like someone may have cleaned some of the carbon deposits out of the exhaust port but just got the stuff close to the piston and left the rest of it. if you take the cylinder off, and it turns out to be re-usable, clean the rest of that out.
SteveSr
Addicted to ArboristSite
That wear pattern sure looks odd. The FW side of the piston is well worn but the PTO side is near pristine. I would guess that there was/is a FW side seal leak that raised cylinder temps on the FW side causing the wear. This family of saws is known for FW side crank seal leaks.
If you do have to tear it down you need to determine if the cylinder and piston are OEM or AM. Pressure and vac test is in order first.
If it was me I would take the top off, crank the saw, and while it is idling squirt some starter fluid down behind the fly wheel. See what happens. If nothing it is not likely a seal leak on that side. If it revs up or kills it that would throw a flag. Make sure the screws holding the jug on are all tight as well..
If you want to do the clutch side you will have to take the clutch off to spray that seal. Still easier than all that vac/ pressure BS. Even if it has a leak you are going to have to find it.
All that vacuum pressure tells you to go get some soapy water or something to see bubbles. And, you will have to actually pull the flywheel to do that.
If you want to do the clutch side you will have to take the clutch off to spray that seal. Still easier than all that vac/ pressure BS. Even if it has a leak you are going to have to find it.
All that vacuum pressure tells you to go get some soapy water or something to see bubbles. And, you will have to actually pull the flywheel to do that.
crank seal leaks don't always show with pressure, which is all you're going to get by running the engine and using soap. if they are leaking on the exhaust stroke, which is more common and which is what pulls in air from a leak, the soap test isn't going to show that from the outside. but whatever. he should absolutely do the vac and pressure test after reassembly anyway, so how much more trouble is it to do it before?
Good Lord.
SteveSr
Addicted to ArboristSite
Looks like something has been festering for a while. Cylinder is probably salvageable with proper transfer removal and cleaning. Piston should probably be replaced. Probably not a good idea to run it like this.
This is an excellent example of why it is always a good idea to pull the muffler and inspect the piston of any 2-stroke that you are considering buying.
This is an excellent example of why it is always a good idea to pull the muffler and inspect the piston of any 2-stroke that you are considering buying.
Thanks for the picture. Some wear, but I would run it first. Might be OK for now. The machine marks going across are what you are looking for.
EastCoastGreg
Excuses are crap, Reasons matter.
I would also run the saw and see if runs strong. If it does, then take the top end apart clean it all up inspect the rubber parts, mainly the impulse line. I would not shotgun parts at a saw before even running it in some wood though. I agree after seeing the piston it could use some attention, but not necessarily replacement. Crankcase leaks present pretty obviously when running(most of the time), as erratic idle based off saw position, and inability to keep a saw in tune. This is by no means a comprehensive list, and not intended to be.
I have successfully repaired saws without replacing lots of parts. Some as simple as deburring any markings on pistons, replace a ring(s) if damaged. One saw in particular would idle but not rev, due to compression being low, I replaced a ring, cleaned and dressed piston. Lightly ball honed the cylinder, was a non-plated cylinder, and the saw has 160psi now. Doesn’t always work that way but it can be done if the scoring is less than .001” (.025mm). Just some thought for the OP. It can produce a well running saw for a homeowner cutting firewood and property maintenance. Personally, I would not do it on a production saw or if I was being paid to fix a saw. Just some thoughts.
I have successfully repaired saws without replacing lots of parts. Some as simple as deburring any markings on pistons, replace a ring(s) if damaged. One saw in particular would idle but not rev, due to compression being low, I replaced a ring, cleaned and dressed piston. Lightly ball honed the cylinder, was a non-plated cylinder, and the saw has 160psi now. Doesn’t always work that way but it can be done if the scoring is less than .001” (.025mm). Just some thought for the OP. It can produce a well running saw for a homeowner cutting firewood and property maintenance. Personally, I would not do it on a production saw or if I was being paid to fix a saw. Just some thoughts.
EastCoastGreg
Excuses are crap, Reasons matter.
If you run the saw and it runs terrible and the seller said it ran well you can return it. If you tear it all apart then try to return it, probably not gonna have much luck. Again just a thought
In your video look closely at the 2 nd cycle of the piston there is a large burp of compressed air pushing bubbles of oil up from the crankcase as the piston moves, that loss of compressed air is affecting the impulse to the carb a fair bit ,that affects the carb`s ability to pump fuel. My take on that is that the bottom of the intake side piston skirt is worn a good bit, best to pull the cylinder off and give it a close look over.
seems like a weird wear pattern. it's like the oil holding lines just end halfway across the port.
