Stihl 026 Compression

[Stihl051master]

Hello,
I picked up a used Stihl 026 today for a heckuva good price. The guy said he thought it needed a new cylinder and piston, because it appeared to have some very light scuff marks on the piston, which you could see after removing the muffler. Looking through I really didn't see anything out of the ordinary. I should point out I'm no chainsaw expert, but worked at a motorcycle/snowmobile shop and rebuilt a TON of 2 stroke engines. My question is what should the compression reading be? I tested it as soon as I got home, and it read 170lbs. That seems like an excellent reading to me, and since I don't see any horrific scoring leads me to believe the p/c are fine. The guy runs a small shop, and is super meticulus about his stuff, so I am thinking he just wanted the saw back to perfect. In my bag-o-parts that came with the saw (it was partially disassembled) he accidently gave me the wrong carb, so I couldn't fire it up. Do you guys think I should just rebuild the carb, put new fuel lines and an air filter on, put her back together and run it???

 

[blsnelling]

That's fabulous compression. I consider 150 good.

 

[Stihl051master]

When I go back and exchange carbs, of course, and learn to spell meticulous (?) correctly.....

 

[Stihl051master]

That's what I was thinking. We used to do the "thumb test" on the two strokes. Seems like as long as there was about 100 lbs it was good enough to run (although maybe not to its full potential. Should I be worried about the scuffing on the piston and replace it, or just keep a close eye on the saw and run it? Like I said I'm no chainsaw expert, but on any other 2- stroke I wouldn't worry...It's just that the saws run so much higher r's...

 

[blsnelling]

Can you upload a pic for us?

 

[Stihl051master]

I can try if I have time tomorrow. I'm on crappy dial-up and accidently deleted my photo editing software so the pic will be bigxhuge.

 

[blsnelling]

I can try if I have time tomorrow. I'm on crappy dial-up and accidently deleted my photo editing software so the pic will be bigxhuge.

Email it to me and I'll resize it. I'll PM you my email addy.

 

[Lakeside53]

That's what I was thinking. We used to do the "thumb test" on the two strokes. Seems like as long as there was about 100 lbs it was good enough to run (although maybe not to its full potential. Should I be worried about the scuffing on the piston and replace it, or just keep a close eye on the saw and run it? Like I said I'm no chainsaw expert, but on any other 2- stroke I wouldn't worry...It's just that the saws run so much higher r's...

100? na.. a small saw won't even run...

170 isn't unusual and is good. However, test it after it's been run as cold it may be due to oil in the cylinder.

Just run it...

 

[Stihl051master]

Thanks for the info, Lakeside. I will get the saw running and then keep an eye on the reading. What should I look for as a minimum reading if I'm picking up a used saw? 140-150?

 

[stihl025]

However, test it after it's been run as cold it may be due to oil in the cylinder.

Andy,

My MS260 came in about 160 when it was cold. My 025 came in about 140 cold. Is it better to test the compression after it has been running or dead cold?

 

[Lakeside53]

6 of one... it's just a number that should be replaced with a red, yellow, green indicator

If you KNOW the saw and have run it previously, either way is fine. If you get an unknown non-running saw, and get a high compression reading, be cautious.

 

[masiman]

....If you get an unknown non-running saw, and get a high compression reading, be cautious.

LS, why would you be particularly cautious with this scenario?

 

[Lakeside53]

Because the high compression may just be congelled oil in the cylinder...
and..

If want a saw it read 170, I can just squirt in some 90wt though the spark plug hole...

 

[Stihl051master]

I did test the saw "dry" - it hasn't been run in at least a year and all the fluids were drained. I just checked it when I got home to see what it had, as I was told from the guy I got it from it needed a new p/c. I've seen extra-high readings due to a heavy build up of carbon also...

 

[Stihl051master]

Here's a pic of the piston through the exhaust port

 

[blsnelling]

There's very little detail in that picture. Also, it's way over exposed on the piston, blowing out the colors, hurting resolution worse. Try taking another without flash using ambient light.

However, I have to say it doesn't look good. It appears as though there is a lot of scoring on the piston.

 

[Lakeside53]

There's a piston in there?


Take your orignal high res picture, cut out the exhaust port and post that... (even "paint" will do that)

 

[stihl025]

However, I have to say it doesn't look good. It appears as though there is a lot of scoring on the piston.

How much scoring is acceptable? I have never done a P&C rebuild so I have never had the jug off. On occasion, I will pull the muffler to make sure everything "looks" okay. I noticed that one of my buddy's saws (029) has some scoring on the piston, but looking through both the carb and muffler ports at the cylinder walls, they appear to be unharmed. There are slight scratches associated with wear. Nothing deep, very minor. How can the piston have some scratches, but not the wall? There was alot of carbon build up on the exhaust port. Do you think that the build up could hit the piston and scratch it without causing damage to the cyl. wall?

 

[Lakeside53]

yes.. carbon scratching will damage the piston and can leave the cylinder coating alone.

Light PISTON scoring has little effect on the operation. If the scoring extends up into the rings, and the rings have machining scoring, time to do something about it.

 

[stihl025]

yes.. carbon scratching will damage the piston and can leave the cylinder coating alone.

Light PISTON scoring has little effect on the operation. If the scoring extends up into the rings, and the rings have machining scoring, time to do something about it.

Thanks, Andy.

The rings look fine, and like I said, no cyl. wall scratches. I usually don't fix something that is not broke. I will help him keep and eye on it, though.

I do want to clean all of that carbon build up on the exhaust port. I have seen some threads using oven cleaner, brake cleaner, gas-mix. What do you think is best, and where should the piston be in the cylinder at the time of cleaning?