Stihl 026 Top End Rebuild Compression Problem

[RabbitFarmer]

I was refreshing my 026 Stihl saw (new fuel line, filter, impulse line, new coil and wires, air filter, intake manifold, new jug and piston, cleaned out carb etc). At first I bought a jug and piston from amazon (https://www.amazon.com/dp/B07X2HFXJG?psc=1&ref=ppx_yo2_dt_b_product_details), but the compression wouldn't get even above 30PSI and it obviously wouldn't run so I ordered a Duke kit from Ebay (https://www.ebay.com/itm/283552679470) and I still had the same exact problem. I have done top end rebuilds on 4 stroke motorcycles, but this is my first 2 stroke. Is it possible I'm missing something? It seems super easy just align the piston with arrow to exhaust and align the ring end gaps up with the slots on the piston. The saw is the older style 026 and I had to specialty order the old style fuel line in order to get it to fit properly so I'm thinking its possible that these kits aren't compatible with the old, old 026 model but that seems weird to me. The other possibility is that both kits are junk but the Duke kits seem to have good reviews on this forum and others so I don't want to just jump to that conclusion. I've tried running the comp test with carb on, off, full throttle, exhaust off, every combination/variation I could think of with both kits with no difference in results. I've also tried spraying WD40 on the sparkplug and decompression threads to see if any air is bubbling out and it looks air tight there. I've also tried a little WD40 in the bore to help seal rings a bit and that didn't change anything either. What else can I try? any advice would be helpful thanks.

 

[pioneerguy600]

We don`t really trust the compression testers most often used on chainsaws. An automotive type compression tester will always read very low on these small displacement engines. A dedicated small engine compression gauge with check valves in both ends of the hose is what gives the correct compression numbers.

 

[Vintage Engine Repairs]

it’s probably your compression tester. Take the after market top end off off and put your OEM back on and a new set of OEM rings if you feel compression is low. A coil isn’t a wear item as such, don’t replace it just because you are doing a service on lines etc. If you have spark issues then sure, but go OEM.

 

[Maintenance supervisor]

Use the spark plug hole, possibly with a crush washer.

 

[RabbitFarmer]

it’s probably your compression tester. Take the after market top end off off and put your OEM back on and a new set of OEM rings if you feel compression is low. A coil isn’t a wear item as such, don’t replace it just because you are doing a service on lines etc. If you have spark issues then sure, but go OEM.

I had a no spark issue, that's why I replaced the coil, I have a good spark now. I might try to find a compression tester for chainsaws as my next step.

 

[RabbitFarmer]

We don`t really trust the compression testers most often used on chainsaws. An automotive type compression tester will always read very low on these small displacement engines. A dedicated small engine compression gauge with check valves in both ends of the hose is what gives the correct compression numbers.

Can you link one of those to me my tester does have a check valve and worked fine for my motorcycles.

 

[Bob Hedgecutter]

Can you link one of those to me my tester does have a check valve and worked fine for my motorcycles.

The thing is- do you NEED to know what the compression reading is?
Some OCD types get very hung up on numbers, only tune with a tach and must own every OEM specialist tool known to have been invented, to mess around with a handful of saws.
Generally- a new top end (even the low quality Aftermarket ones) will make sufficient compression to run the engine if everything else is correct.
If you put it all together and it runs- happy days. If it doesn't (with a new top end) odds are compression is not your problem.
Assembly compression readings are so so and should become stronger once the cylinder walls and fresh new rings have become good friends.

 

[pioneerguy600]

There are many makers of small engine compression testers, most important it that they have a schrader valve in the end that screws into the sparkplug hole. The link below is a dedicated tester but there are many other makers like OTC , Snap On etc,

https://www.amazon.ca/Products-Small-Engine-Compression-Tester/dp/B073FGKFG7

 

[RabbitFarmer]

The thing is- do you NEED to know what the compression reading is?
Some OCD types get very hung up on numbers, only tune with a tach and must own every OEM specialist tool known to have been invented, to mess around with a handful of saws.
Generally- a new top end (even the low quality Aftermarket ones) will make sufficient compression to run the engine if everything else is correct.
If you put it all together and it runs- happy days. If it doesn't (with a new top end) odds are compression is not your problem.
Assembly compression readings are so so and should become stronger once the cylinder walls and fresh new rings have become good friends.

Well I spent around 2k and 200 hours of my life chasing issues on a motorcycle that I had to store in my bedroom during college only to find out the aftermarket piston I bought wasn't making good compression (It was the first engine I ever put my hands on). So now when I work on an engine and the compression tester says 20 PSI and it wont run, and I know its getting spark, and when I spray ether on the filter it barely chugs for a tenth of a second I'm hesitant to move forward without establishing that good compression exists. I have tried running it with 2 different top ends multiple times, 2 different carbs and at this point I don't know what else to try besides the fact that both top ends said 20 PSI on my tester. I will find a tester with a Schrader valve. I cant go back to old piston and jug because they are toast.

 

[huskihl]

Are you certain the case is an 026 and not an 024?

 

[Maintenance supervisor]

Well I spent around 2k and 200 hours of my life chasing issues on a motorcycle that I had to store in my bedroom during college only to find out the aftermarket piston I bought wasn't making good compression (It was the first engine I ever put my hands on). So now when I work on an engine and the compression tester says 20 PSI and it wont run, and I know its getting spark, and when I spray ether on the filter it barely chugs for a tenth of a second I'm hesitant to move forward without establishing that good compression exists. I have tried running it with 2 different top ends multiple times, 2 different carbs and at this point I don't know what else to try besides the fact that both top ends said 20 PSI on my tester. I will find a tester with a Schrader valve. I cant go back to old piston and jug because they are toast.

How far are you from Dike Iowa? There's a fellow with a chainsaw museum there that has forgotten more about chainsaws than most of us know.
Being that you have 2 separate and different kits something is definitely weird.

 

[merc_man]

Have you done a pressure vac test for air leaks?

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[Bob Hedgecutter]

Well I spent around 2k and 200 hours of my life chasing issues on a motorcycle that I had to store in my bedroom during college only to find out the aftermarket piston I bought wasn't making good compression (It was the first engine I ever put my hands on). So now when I work on an engine and the compression tester says 20 PSI and it wont run, and I know its getting spark, and when I spray ether on the filter it barely chugs for a tenth of a second I'm hesitant to move forward without establishing that good compression exists. I have tried running it with 2 different top ends multiple times, 2 different carbs and at this point I don't know what else to try besides the fact that both top ends said 20 PSI on my tester. I will find a tester with a Schrader valve. I cant go back to old piston and jug because they are toast.

Agreed, but on the motorcycle you only replaced the piston- not the entire top end. If you are trying to take a reading through the decompression valve fitting- you will never get a full compliment of the contents of the combustion chamber to get a good reading.
I don't suppose you have another chainsaw that does run? If you did you could test it with your gauge and see if it gave you around 30psi reading- thus eliminating lack of compression.
Was kind of common sense the original cylinder and piston were toast- that is why I never suggested putting it back on.

 

[Chevboy0167]

Use the spark plug hole, possibly with a crush washer.

Try this first. In the pics, you have the compression tester in the decompression valve hole. Plug that hole with the decompression valve or a decompression valve eliminator plug, remove spark plug, drop a table spoon full of 2 stroke oil down spark plug hole, turn and twist saw around to distribute oil to cylinder wall, pull recoil rope over 4-5 times slowly with spark plug hole open, install compression gauge in spark plug hole and retest.

 

[RabbitFarmer]

Ok I dug through my tool bucket and got the fitting for the sparkplug thread. Testing with the nut/washer in the decomp thread and still exactly the same 20 psi reading. I also tried to measure the Squish band clearance with .023in (.6mm) diameter solder and it did not contact the solder at all. I had to wrap it/stack it 3 high to contact the top of edge of the jug. I've never measured the squish clearance before so take that with a grain of salt, but its not under .023 I can grantee that. I'm done for the night you, guys are rock for trying to help me out.

 

[sean donato]

have you checked crank seals? Having the engine sealed up properly is vitally important on a 2 stroke. What made you decide to tear it down in the first place? What is wrong with the original topend? Pics go a long way, and will help to determine what other issues you hade before you dove into too end replacement mode.

 

[sean donato]

Ok I dug through my tool bucket and got the fitting for the sparkplug thread. Testing with the nut/washer in the decomp thread and still exactly the same 20 psi reading. I also tried to measure the Squish band clearance with .023in (.6mm) diameter solder and it did not contact the solder at all. I had to wrap it/stack it 3 high to contact the top of edge of the jug. I've never measured the squish clearance before so take that with a grain of salt, but its not under .023 I can grantee that. I'm done for the night you, guys are rock for trying to help me out.

Seeing this post after I posted my response, brings us back to first making sure you have an 026, and second what's wrong with your oe top end?

 

[RabbitFarmer]

Seeing this post after I posted my response, brings us back to first making sure you have an 026, and second what's wrong with your oe top end?

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.

 

[kenton.a.frank]

Just FYI there were a few different top ends. If you have a 024 case and crank with a 026 top end it won’t reach the top of the stroke as the 024 was slightly shorter. Also, there were two different displacements in 026 cylinders. If you use the smaller one on the larger cylinder it will feel kinda right but compression will be terrible.


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[RabbitFarmer]

Just FYI there were a few different top ends. If you have a 024 case and crank with a 026 top end it won’t reach the top of the stroke as the 024 was slightly shorter. Also, there were two different displacements in 026 cylinders. If you use the smaller one on the larger cylinder it will feel kinda right but compression will be terrible.


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I think originally it had the smaller displacement. Its the old 026. The kits I have I think are 44.7mm while the original is 44mm, but I was thinking they would be the same stroke length.