Stihl 026 rebuild

[Brewz]

Was working outside around the house today and it decided to rain.
So I wandered into the garage to tinker with stuff.
I dug out my dremmel and decided to do a bit of work on the 026 cylinder.

First of all I measured the exhaust port width with a piece of paper by trimming it to the width of the port when following the curve of the cylinder.
I got 23.4mm (53.18%) of the bore width.
From what I have read on here, 70% is an upper limit depending on the motor being ported so I decided to stay conservative on my first attempt.
I measured the side skirt of the piston that covers the ports and it was 31.3mm. I wanted to keep some meat in there to stop gasses bypassing the piston skirt to the exhaust port so I aimed for a width of 28mm (63.63%)
I ended up with 27.4mm (62.2%) and was happy with that.
I left the intake and exhaust ports top height standard and lowered them both by about 1mm.
I removed about 1 to 2mm of material from all around the exhaust port to bell it and tidied up the intake, smoothing it out and rounding curves to help aid air momentum. I didnt't take too much out as it is thin walled and I figure the flow will be restricted by the carb and intake boot no mater how much metal I remove.
Then I moved to the transfers and ground down the square edges, trying to build a smooth transitional surface to get the air flowing into the transfer faster and smoother. I extended the leading edge facing the intake port and rounded out all the curves.
I didn't touch the top of the transfers. Firstly I dont have a bit I would be comfortable using on them without risking damage and also, I figure getting the fuel/air mix flowing in smoothly is the main aim and leaving the top standard will keep velocity high.

I would love to hear some critique from more experienced porters on my job thus far, good or bad!
The lines in the exhaust port below are me feeling the surface out with a screw driver, not cracks

 

[drf255]

You do some very nice grinding. Beautiful stuff.

I'm not a pro, but a few points.

The more you grind out anything below the upper transfers (lower transfers and piston Windows), the bigger you make the case volume and the lower the case pressure will become. Hogging the lowers out reduces charge pressure and velocity, but you did less than a lot do at first (including me).

Lowering the exhaust floor may have been a mistake, but at .040 you should be ok. Even if the piston crown drops below it stock (IIRC it doesn't-can check later) most of the exhaust has already been vented way before it gets there. The biggest issue with the exhaust floor is that it seals itself against the skirt at TDC. You have to make sure you are not freeporting under the skirt at TDC. A little is ok, and at .040 I think you should be OK. But I wouldn't do it again.

Lowering the intake floor affects the intake timing. Lots of different opinions on that here. The intake floor is where case compression starts. You should have had around a 74* closing point stock. You're probably still ok. Most guys don't want more than 80*, but then again that varies by saw model and builder opinion. I'm not sure what .040 translates to in degrees. The piston doesn't move much at TDC and BDC per degree of crank rotation.

Most important is to bevel/chamfer all of your portwork when done. That means bringing a smooth edge on all the areas where the plating meets the port wall. Especially where you did grinding. A small ball works great for that. I use a diamond coated ball. Many use stones.

You made some really nice ports. I've seen worse work from some pros here.

Now toss a degree wheel on that sucker.

 

[Brewz]

Now toss a degree wheel on that sucker.

I don't have one

Thanks for the kind words...... I am very happy with how it turned out.

With the transfer lowers, I was considering taking a bit more out but wont now. Most of the material I removed was rounding the squared out edges. The rest of it is just a minor surface polish. I didn't do any digging and I am glad I didn't now.

I remembered I marked a light scribe mark on the piston at the base of the exhaust port with the piston at TDC to see how much room I had to move.
I just went and checked it again and I actually only removed about .020 and there is a good 0.100" of piston skirt below the base of the exhaust port when at TDC. My main aim was to widen the ports. Intake came down the same.

Some of the factory bevel is still there and I did touch the edges back up with a small stone.

 

[drf255]

There's a PDF of a TAVIA one on here somewhere. Hopefully someone will be around with it shortly. You print it out and glue it to something firm. Tada, you've got a degree wheel.

That's good news on the exhaust floor.

I'm only stating what I've been taught/told. There's more than one way to skin a cat.

Some builders here swear by a high intake floor and even use epoxy to raise it on some models. Sone think it's BS and like more intake. Many of their saws run equally well.

You definitely have your head I the right place. You measured and checked everything before grinding. Kudos to you.

One builder here makes a strong 026 and does little to the jug. He sets squish at .017, advances ignition timing .020 keyway, mods the muffler to a hole that's 1" square. He widens the intake and blends in the large chamfers on the exhaust roof so there's some more flow without affecting the exhaust opening point. He feels that the large factory chamfer there is unnecessary. He uses a stock piston and does nothing to the upper transfers or the combustion chamber. He cleans up the piston Windows.

The 026 of his that I have pulls 200psi, maintains the ability to use a stock piston, and runs remarkably well.

Can't wait to see yours run.

 

[Brewz]

Yeah I am hanging out to crank it as well but I picked a bad time of year to order parts...... some of them international orders.
It could be a couple more weeks yet

I would love to get hold of the degree wheel PDF if someone has it. Might even try google.

When i get it together I am going to get my dad to fuel his stock saw which is the same and set up a race. Will video!

Now, something I still dont know........ what idle and max RPM should I shoot for when tuning it? I believe stock max rating is 14000 rpm?

 

[drf255]

You should vac test the saw once you have the jug attached.

Idle set to factory spec.

High RPM set point from factory went out the window when you touched the saw. Set it rich at first, then lean it out so it burbles out of wood and the burble goes away when you cut at WOT.

The tach reading will be for curiosity only. It should 4 stroke out of the cut at a higher rpm than the factory. Sometimes it won't, and the saw will just be stronger.

 

[Stihlofadeal64]

Isn't .5mm the same as .020?

That would put him back where he started.

Brewz, they make brass shim material that comes in multipacks of different precise thicknesses. I've used it a few time for that final squish adjustment.

You are correct on the .020 measurement. I actually am good at 175psi as he posted. If he wants to tighten the squish more then your suggestion will probably work. I'd be a happy man at 175psi actually. So where are you ideally looking to get the squish set at and what compression numbers are you getting on your saws? I am not being sarcastic or cynical just curious.

 

[Brewz]

You should vac test the saw once you have the jug attached.

Idle set to factory spec.

High RPM set point from factory went out the window when you touched the saw. Set it rich at first, then lean it out so it burbles out of wood and the burble goes away when you cut at WOT.

The tach reading will be for curiosity only. It should 4 stroke out of the cut at a higher rpm than the factory. Sometimes it won't, and the saw will just be stronger.

Ok that makes sense.

I guess I should run a bit more oil than normal.
My standard mix is 98 octane fuel and 50:1 Stihl Ultra HP

Will that do for a ported saw?

I might need to start an oil thread

 

[Canadian farm boy]

Was working outside around the house today and it decided to rain.
So I wandered into the garage to tinker with stuff.
I dug out my dremmel and decided to do a bit of work on the 026 cylinder.

First of all I measured the exhaust port width with a piece of paper by trimming it to the width of the port when following the curve of the cylinder.
I got 23.4mm (53.18%) of the bore width.
From what I have read on here, 70% is an upper limit depending on the motor being ported so I decided to stay conservative on my first attempt.
I measured the side skirt of the piston that covers the ports and it was 31.3mm. I wanted to keep some meat in there to stop gasses bypassing the piston skirt to the exhaust port so I aimed for a width of 28mm (63.63%)
I ended up with 27.4mm (62.2%) and was happy with that.
I left the intake and exhaust ports top height standard and lowered them both by about 1mm.
I removed about 1 to 2mm of material from all around the exhaust port to bell it and tidied up the intake, smoothing it out and rounding curves to help aid air momentum. I didnt't take too much out as it is thin walled and I figure the flow will be restricted by the carb and intake boot no mater how much metal I remove.
Then I moved to the transfers and ground down the square edges, trying to build a smooth transitional surface to get the air flowing into the transfer faster and smoother. I extended the leading edge facing the intake port and rounded out all the curves.
I didn't touch the top of the transfers. Firstly I dont have a bit I would be comfortable using on them without risking damage and also, I figure getting the fuel/air mix flowing in smoothly is the main aim and leaving the top standard will keep velocity high.

I would love to hear some critique from more experienced porters on my job thus far, good or bad!
The lines in the exhaust port below are me feeling the surface out with a screw driver, not cracks

View attachment 473296

View attachment 473297

View attachment 473298

View attachment 473299

Your upper transfer ports look like they are real close to your intake port. You will want to check and see where your piston ring ends are. You don't want to catch a ring in the upper transfer ports. If your ring ends happen to interfere with the upper transfers you can possibly turn the piston around 180*

Edit; I looked at the pics again. I'm using my phone with a scratched up/broken screen. Your upper transfers and intake port look fine. Please disregard what I had first said. I need more coffee...lol

 

[Duane(Pa)]

Oil thread? No thanks! All I can say is 50:1 is about exhaust emmisions, not protection of moving metal....

 

[drf255]

You are correct on the .020 measurement. I actually am good at 175psi as he posted. If he wants to tighten the squish more then your suggestion will probably work. I'd be a happy man at 175psi actually. So where are you ideally looking to get the squish set at and what compression numbers are you getting on your saws? I am not being sarcastic or cynical just curious.

I'm cutting the chambers to lower the exhaust, so I'm winding up at around 225-240 psi.

I'm at .020 squish.

Lol, you're not being cynical at all.

Ok that makes sense.

I guess I should run a bit more oil than normal.
My standard mix is 98 octane fuel and 50:1 Stihl Ultra HP

Will that do for a ported saw?

I might need to start an oil thread

32:1 in my opinion.

 

[Stihlofadeal64]

I'm cutting the chambers to lower the exhaust, so I'm winding up at around 225-240 psi.

I'm at .020 squish.

Lol, you're not being cynical at all.


32:1 in my opinion.

I've read your insight and noted it. I'd be interested also to know how many work saws are also having longevity in the field with these types of numbers (day in and day out). Not saying it is not possible at all (I really can't say). But I do know there are a load of saws in the field with 175ish working day after day. Can you elaborate?

 

[drf255]

I cannot elaborate at all.

I'm sure it's like everything else in life.

Fast and hard don't last as long.

I'm just having fun.

 

[gritz]

Looks like you're moving right along, Brewz! I think I have the picture of the degree wheel on my other computer. I can look tomorrow. Worst case scenario, I can take a picture of mine and post that up. Really, you just need a high resolution picture of a degree wheel. You can do a search on google images and just find one you like. I purchased one, but it's really not necessary. I can't remember exactly what my numbers were on my saw, but I think I went too far on the exhaust. It makes great RPMs, cuts really well, but really doesn't like to be pushed too hard. I think a little less on the exhaust might have saved a little torque, but who knows. I used a dowel rod wrapped in sandpaper to smooth out my exhaust port. Not sure if I posted anything about that in my port thread. I use a 1/4" air grinder and a 1/4" dowel rod. I use 400-1000 grit paper to get it nice and smooth. It's probably not necessary, but I'm like a magpie and like shiny things, lol. If it looks fast, it is fast, right!? I'm not expert on porting, but I've worked on a lot of those 1121 series saws - a lot. Definitely nice little saws. If you need parts, definitely post up and ask. Lots of us have several of those saws laying around. I think I have three running 026/260 saws and parts to make one more probably. Keep up the good work!

 

[Brewz]

I don't see a problem with pushing the limits as long as you dont expect it to be as reliable as a low compression saw, which is obviously the case with what drf has done.

I have a mate who builds 600+ HP rotary engines in little Mazda's that rev like a chainsaw and are slightly terrorfying to ride in.
He accepts the fact that pusshing a small motor beyond its usual limits will result in regular breakages but fixing it when it breaks is half the fun!

My dad told me while I was cutting large amounts of fallen trees on his property that he didn't want me to wear out my saw. I told him it was fine cause if it breaks I can pull it apart and fix it, and probable do some more modifications in the process.

Some of us are just like that.

For my 026, I want to keep the compression under 200 psi as it will be a work saw. Most of the work I am doing is to clean up and improve on what is there as opposed to modification. It's a bit of a learning experience before I strip my 066 and start on it.

 

[Macman125]

Ok that makes sense.

I guess I should run a bit more oil than normal.
My standard mix is 98 octane fuel and 50:1 Stihl Ultra HP

Will that do for a ported saw?

I might need to start an oil thread

I would fatten up the oil mix a little bit. I run 40:1 using castor 927.

 

[Macman125]

Do you have any more pics of the work you did to the cyl? I have a 260, I am looking to at least get rid of the base gasket.

 

[Brewz]

Do you have any more pics of the work you did to the cyl? I have a 260, I am looking to at least get rid of the base gasket.

I dont but I can take more if you really want. Wont show much more that I already posted above.
If your 260 is the same as my 026, you wont be able to delete the gasket without some machining work on the top of the cylinder.
Mine measures .009 squish with no gasket.
I have ordered .010 gasket material to get up around the 18 to 20 thou squish mark.
I am not too worried about the tight squish as I will watch for carbon buildup if it happens but doubt it with a good tune and good oil.

I am no porting expert and have started out with this one after a lot of reading time on here, making as little change to timing as I could. My main objective is to increase airflow and make the saw stronger, not rev higher.

 

[Macman125]

I have access to a friends machine shop. Machining the jug really isn't a big deal. I already have done a nice muff mod, advanced the timing and replaced the fixed jet carb with a wt194. From my understanding, everything is the same between the 026 and 260 except; the 260 has a bore of 44.7 while the 026 has a bore of 44mm. The 260 also received an upgraded clutch driven adjustable oil pump as compared to the 026 having a direct drive. All parts are interchangeable between the two I do believe though including the p/c.

 

[Brewz]

Hey Gritz.

I would really REALLY love a nice factory sprocket cover!

Mac...... check out this post...... might be what your after.

http://www.arboristsite.com/community/threads/ported-metal-badge-026.289968/#post-5657921