Stihl 026 rebuild

[drf255]

So your exhaust opens at 95*?

Transfers at 122*?

I'm out of my league here. Not sure about the numbers and durations.

Interested to see what others think.

 

[Brewz]

Yes and yes..... when the piston is on the way down from TDC

 

[redfin]

Are you trying to reduce your squish?

 

[drf255]

What's your squish at without the gasket?

I'd raise those transfers if you have the ability to.

 

[Brewz]

Squish is at .009 with no gasket. I will be setting the squish at about .019 when my .010 gasket material shows up.

At this stage I am interested to know if the numbers this setup has are good or bad, what could be done to improve it and why!

 

[redfin]

Dropping your jug is gonna help a tad lowring your ex. I would also raise your transfers if you can. I wouldnt touch the intake. You can always go back in if you need more int.

 

[Brewz]

Just for my personal education, what will raising the upper transfers do for the saw?
Should I just raise then the .010 I plan to lower the jug to maintain timing or should I go a bit further and blend the edge off a bit at the top?

If I know an optimal degree value between exhaust and transfer (remembering I want torque not RPM's) I can mark it and port to the line.
Raising them is open to interpretation by the nut behind the butt with a carbide bit in hand

 

[drf255]

Let's start with the absolute truth that the factories could build a faster saw than anyone of us. But they want a fair amount of power with fuel efficiency, a certain emissions limit and reliability/longevity. We are just removing parts of the last three for more of the first one.

What did stihl do to the MS260 when the had to meet stricter emission requirements and still make the same power output? They simply increased the cylinder diameter from 44 to 44.7. Something we can't do.

The only way to make more power is to burn more fuel, or compress what you're burning more to squeak more power from the explosion.

Now that we understand that, we can get into what will make more power.

The transfers are lower than is optimal for power from the factory.

 

[drf255]

The transfers are set so that they push fresh change into the cylinder and the charges pushes the exhaust out.

There's a certain amount of fresh charge dilution by exhaust gasses. If you push too much fresh charge in, you push all of the exhaust out and even some fresh unburned mix. So you will make great power, but throw fuel out. The saw will be a hog. Push less in, you will have more charge dilution, less power and better fuel efficiency.

The key is finding the right balance. This is where we stand on the shoulders of giants, the guys who have done the experiments themselves and have wasted their time and money to try things out.

 

[drf255]

So, if you raise your transfers (up to a point) they will supply more fresh charge for combustion.

This is where blowdown comes into play. If you raise them too much, the exhaust will flow down them and you will get less charge or an engine that doesn't run at all.

 

[drf255]

So now that you see the theory above, the 026 runs well with the transfers at 118. You could even go 116 because your exhaust is so high.

On my last build, my exhaust was at 104 and the transfers at 119. So the same amount of blowdown for you would be at a transfer height of 110. I wouldn't do that.

My point is that 118 will be safe

 

[drf255]

Oh, and there's no distance that equals degrees. As the crank angle changes, so does the distance that the piston moves for each degree of crank rotation.

And the way you do it is with the degree wheel. You drop the piston to BDC, put a ring in the bottom of the cylinder, install the jug and snug up two bolts place diagonally, then rotate the motor over slowly until the degree wheel hits 118. Reverse the rotation back to BDC and remove the jug.

Now the bottom of the ring is at 118. You mark it with a sharpie, and remove the ring. Grind up to the line, but watch the angle of the grind. Go in too perpendicular to the cylinder wall, and you can cut though the jug.

 

[drf255]

Sorry for the lengthy response. I'm just trying to give you an understanding, the way I see it, of port design and timing function.

 

[Brewz]

Thanks for the explanation mate, that all makes perfect sense.
Now that I understand the engineering behind it I will have a go.

I am guessing it is a good idea to try and get the charge entering the opposite side to the exhaust first to get it pushing the spent charge out?

What I will do is wait till I get the new piston and gasket and measure it all up with the components I plan to use.
I had a read of Gritz's porting thread and he had about 20* (97/117 from memory) of blowdown, you have run 15* and I am currently at about 27* which is a lot longer before I start pushing the exhaust gas out isn't it.
I will wait till I get the parts, measure and port to 118.

You can always take some more out, but its hard to put back in

 

[SteveSr]

Looking in my Stihl 026 parts manual, it looks like what they list as a WT-426 ?

The carb is a WT-394. You have a late production 026 Pro.

 

[Matt B]

Enjoying the thread. I've got a few 026's in boxes waiting on rebuilds and my first crack at porting.
Looking forward to seeing the final results and the comparison with the stock 026.
Cheers, Matt.

 

[gritz]

Brewz, I looked and I DO NOT have an extra clutch cover, sorry. Just post up a wanted post in the tradin' post. I'm sure somebody would have one. What bar/chain are you going to run on this beast when you are done?

 

[Brewz]

All good mate, I have actually had a crack at repairing the one I have. I JB weld filled all the holes and pitting and sanded it back. Then painted it with some high temp paint.
It's come up ok and will do for now.

Bar and chain...... At this stage I will use my Oregon 16" 325 063 pro light bar and I have semi chisel and full chisel chains for it.
I find the smaller 325 teeth less grabby when cutting up smaller branches but still smashes through bigger timber.

 

[Brewz]

I couldn't help myself
I mocked up a .010 gasket and marked the upper transfers at 117*
I should have taken a picture but the factory transfers were definitely angled by about 1 - 2* down towards the intake side, making them open at the intake side first.
I extended and rounded the port towards the intake side and tried to keep the slightest angle downwards to encourage the fresh charge onto the intake side, pushing the spent charge out.

I am fairly happy with how it turned out. Would have been easier with a 90* cutter.

 

[drf255]

Nice job.