Stihl 026 Top End Rebuild Compression Problem

[sean donato]

Well the cover says 026 and I would be crazy surprised if my father in law switched the covers out. I don't know what else to look for in terms of 026 vs 024. He doesn't really like wrenching and said he hasn't touched the saw the whole time he has had it. The saw had a no spark issue and the jug/piston had some scoring nothing crazy, but I figured I might aswell replace it along with all the rubber parts on the carb etc. Sadly I'm dumb and already tossed the originals. As far as the seals I honestly don't know how to check them besides visually and I don't see how they would affect compression, but to be sure the compression is truly without a doubt the problem I probably need to get a Schrader valve equipped compression tester.

The compression I'd directly related to how much air/fuel the crankcase pumps into the cylinder. Bad crank seals will create an air leak, and not allow the fuel/air mix to transfer into the cylinder. You should look up a you tube video of a piston port 2 cycle and see how it work, then you will understand better why everything needs to be right in the top and bottom end.
How does it feel to pull over?

 

[kenton.a.frank]

It is the same stroke. As long as it is a 026. I know from experience in my shop about the 44mm vs 44.7… tech cleaned the cylinder and put a new oem piston in a 026 pro… needless to say the compression was terrible with a 44mm piston in a 44.7 cylinder


Sent from my iPhone using Tapatalk

 

[RabbitFarmer]

The compression I'd directly related to how much air/fuel the crankcase pumps into the cylinder. Bad crank seals will create an air leak, and not allow the fuel/air mix to transfer into the cylinder. You should look up a you tube video of a piston port 2 cycle and see how it work, then you will understand better why everything needs to be right in the top and bottom end.
How does it feel to pull over?

Right now I was testing compression without carb or muffler so their wasn't any fuel. It feels smooth, but its pretty easy to pull. I might dumpster dive for the original jug and piston I know they are still there, lol.

 

[Bob Hedgecutter]

What are the odds of an 024 having had a wee accident in a previous life and had a secondhand 026 top cover (resplendent with 026 badge in place) fitted to make it good again?
Would explain everything.

 

[drf255]

Is the case front or side chain adjust?

How many bar studs?

Is the top handle mounted with torx screws or black pan heads?

Does the bottom case rail have a notch for the oiler adjustment?

Can you see the oil pump behind the clutch area? Is it metal or tan plastic? (should be able to see the tab where the oil pickup is. Down and forward)

 

[pioneerguy600]

What ever happened to the old way of doing a quick compression test that was once so common on here? Pick up the saw by the recoil handle and watch how fast the saw winds down. If it truly does have 20 lbs comp the saw would hit the floor very quick like if the compression was truly that low.

 

[Lee192233]

I probably need to get a Schrader valve equipped compression tester.

I hope the tester has a Schrader valve. It holds the pressure on the gauge after pulling it over, correct?

 

[pioneerguy600]

I hope the tester has a Schrader valve. It holds the pressure on the gauge after pulling it over, correct?

Compression testers for small two stroke engines need two Schrader valves, one at the gauge and one at the tip that screws into the engine via the sparkplug hole. If the valve at the fitting end is missing then the hose its self becomes part of the cylinder increasing squish capacity thus lowering compression numbers.

 

[Mad Professor]

Compression testers for small two stroke engines need two Schrader valves, one at the gauge and one at the tip that screws into the engine via the sparkplug hole. If the valve at the fitting end is missing then the hose its self becomes part of the cylinder increasing squish capacity thus lowering compression numbers.

Only reason big ones with one valve are OK is cyl volume is big compared to hose volume. Works on cars/trucks OK, but not precise

 

[Lee192233]

I'm only familiar with my Snap-on tester. It only has a Schrader at the fitting at the end of the hose. It has the long hose but you just have to keep pulling and the pressure will eventually stabilize.

 

[pioneerguy600]

Only reason big ones with one valve are OK is cyl volume is big compared to hose volume. Works on cars/trucks OK, but not precise

On multiple cylinder engines I am mostly concerned with the comp being close between them all, say on a sbc v8 if they averaged around 150 lbs plus or minus a couple lbs I would be ok with that. If two adjacent cylinders showed lower numbers I would suspect a blown head gasket. If one cylinder showed by its self showed low compression I would suspect a defective exhaust valve first or a damaged cylinder/piston if the valve proved out to be okay. A cracked cylinder , seen a few, cracked or broken piston, seen them also. I don`t use a comp tester to reach a specific number on multiple cylinder engines, they make a good diagnostic tool though.

 

[sean donato]

Compression testers for small two stroke engines need two Schrader valves, one at the gauge and one at the tip that screws into the engine via the sparkplug hole. If the valve at the fitting end is missing then the hose its self becomes part of the cylinder increasing squish capacity thus lowering compression numbers.

You only need a valve at the end of the hose. I have several compression gauges, and many test gauges, I like to swap out the gauge to check for consistency. Be hooking the gauge up you are just giving the cylinder a "tank" to fill up. A double valve isn't needed.
And on the compression not being important thing, I've never seen any engine manufacturers say don't worry about the compression. They all have minimum pressure specs, or have a leak down procedure to verify the condition of the cylinder assembly. Telling someone off for doing their due diligence is a tad ignorant. Get the right tool for the job.

 

[pioneerguy600]

You only need a valve at the end of the hose. I have several compression gauges, and many test gauges, I like to swap out the gauge to check for consistency. Be hooking the gauge up you are just giving the cylinder a "tank" to fill up. A double valve isn't needed.
And on the compression not being important thing, I've never seen any engine manufacturers say don't worry about the compression. They all have minimum pressure specs, or have a leak down procedure to verify the condition of the cylinder assembly. Telling someone off for doing their due diligence is a tad ignorant. Get the right tool for the job.

do tell?

 

[sean donato]

do tell?

What am I telling you?

 

[pioneerguy600]

What am I telling you?

Nothing.

 

[Lee192233]

Crickets from the OP.

 

[Bob Hedgecutter]

Crickets from the OP.

Perhaps the Father inlaw fessed up and remembered that the 024 did have a cover swap way back when?

 

[rupedoggy]

Get the original PC. Put something in the spark plug hole to check the stroke. If it is 30mm you have an 024. If it is 32mm then 026. If it is 026 then the aftermarket parts are not right (to much tolerance). Cut the base down (to .020 squish) and it will run.

 

[RabbitFarmer]

haha sorry about late reply I've been swamped at work and was waiting for my fancy Schrader valve compression tester which just came in today. The compression reading on the fancy tester was 70. I'm almost 100% sure compression is my problem now. Now either both my duke top end kit and my potentially sketchier amazon top end kit are both bad or my saw is an 024.

Is the case front or side chain adjust? See picture
How many bar studs? See picture
Is the top handle mounted with torx screws or black pan heads? Black Torx Screws
Does the bottom case rail have a notch for the oiler adjustment? See picture
Can you see the oil pump behind the clutch area? Is it metal or tan plastic? (should be able to see the tab where the oil pickup is. Down and forward) I wasn't sure about this one if its crucial to tell I can spend some more time figuring the answer out.

 

[Bob Hedgecutter]

haha sorry about late reply I've been swamped at work and was waiting for my fancy Schrader valve compression tester which just came in today. The compression reading on the fancy tester was 70. I'm almost 100% sure compression is my problem now. Now either both my duke top end kit and my potentially sketchier amazon top end kit are both bad or my saw is an 024.

Is the case front or side chain adjust? See picture
How many bar studs? See picture
Is the top handle mounted with torx screws or black pan heads? Black Torx Screws
Does the bottom case rail have a notch for the oiler adjustment? See picture
Can you see the oil pump behind the clutch area? Is it metal or tan plastic? (should be able to see the tab where the oil pickup is. Down and forward) I wasn't sure about this one if its crucial to tell I can spend some more time figuring the answer out.

Don't know what all the cryptic questions are for, but do what @rupedoggy says and measure the stoke.
Or list the serial number up here and maybe someone with the access to the US data base can tell you the information logged on to records when it was first sold.