Fitting a Husqvarna 246 Piston to a Stihl 026

[Brewz]

Hi Folks

The idea of fitting a Husky piston into a Stihl has intrigued me since I was told about it when first rebuilding my 066.
I never fully understood the theory of how it gave such good gains until I managed to wrap my head around it in the circle crank thread.

Having decided to fit a Husky 288 piston up to my Stihl 066, and put things into motion to get it done, it got me thinking about my little 026.
Could it be improved on?
It seems the general consensus is to machine the piston top flat and cut the squish band flat to match. Machine the base to get the squish back and they run strong with a lower exhaust port.
This however results in BIG compression numbers which is something I want to avoid in a work saw.

Idea:

Fit a 44mm Husqvarna 246 piston to the 026.
@drf255 and I have ordered one of these...... not sure why HL Supply show 2 rings, perhaps we get a spare?



I have tried to get some dimensions for this piston but had no luck.
@awol said he has one in his race saw but couldn't remember 100% what needed to be done.

I have stripped my 026 down and when the 246 piston shows up, I will bolt it in the 026 and do some measurements.

I am keen to hear peoples thoughts, ideas, concerns and advice on the subject.

If it wont work, the piston will make a nice paper weight on my office desk!

 

[Brewz]

Just for future reference, this is the initial thread I kicked off:

http://opeforum.com/threads/44mm-piston-dimentional-data-wanted-stihl-husky.1945/

 

[Brewz]

I wish I could help you out guys, but the 246 piston is in my 3 cube saw, and we have a race tomorrow!
I do remember that the crown height was lower than on the 026, as I had to drop the jug .040" when going from the 026 to 246. I did do some machining to both of the pistons though, I just don't remember how much. Oh, and the 246 has longer skirts, as it elminated the free porting problem I was having with the 026.

Also for reference, here is awol's post that confirmed my thoughts.

 

[Brewz]

And the port timings on my 026 as it stands now with an OEM jug and Meteor piston.
Zero machining has been done on it.
Ex: 95
Tf: 118
In: 70
Squish .016

The plan is to get the piston, bolt it in and measure the timings again, and decide, with advice from the folks here, where to go with it.

@drf255 feel free to hook in with yours when you get it.
Do you have a fresh jug you could try it with?

 

[Magic_Man]

This is interesting but also funny to me that your building a stihl with a husky piston and I have another thread going right now about building a husky with a stihl piston lol

 

[Brewz]

I guess it all depends on what you want to do or try and what you need to use to get there.

I want to drop the jug to get exhaust timing up over 100* but don't want remove a lot from the combustion chamber.

Awol's figure of .040 makes sense as the husky 288 piston sits 1mm, or .040 lower than the 660 piston.

The effective longer skirt should help with keeping the bottom of the I take shut at TDC as well

Time will tell.

 

[Brewz]

Does anyone have a decent 026 jug sitting in a corner?

I am after one that is lonely and bored with life, and longs for a permenant holiday in Australia cutting eucalyptus.

It will be loved and cared for, and fed a good diet of ethanol free fuel mixed with plenty of nutritious oil.

If you have a cylinder that matches this description, please forward its details to brewzlikesjugs@overpricedaustralian.needs.jug

Edit: that's not a real email, just pm me

 

[David Young]

A 260 has a pin to crown of 20mm. The 246 should be 18.2 but I'll double check. The overall height is a little longer so it should not change timing. But the jug will have to be lowered 1.8mm just to get port timing stock.

 

[ABarrick]

I dont want to rain on your parade but I doubt there will be any benefit till you machine the jug enough to use it. I have every 44mm piston HL has available sitting on my work bench right now hoping to find something that has skirts long enough and wide enough to make a 026 sing and I'm at the giving up stage. The 246 piston is taller overall with slightly wider skirts but the pin location is too high. Till you drop the jug back to stock timing/specs, the skirt only nets you .007 extra lenght over a stock piston. So in theory, you can effectivly lower the jug .007 further with a 246 piston than you can with a stock piston before you free port. I say effectively because the jug is actually being lowered much further. However, now your transfers are way too low, intake is probably too low and the base would be getting very thin. Some guys don't make a big deal about free porting but in my mind it has to be hard on the skirt and I would think you'd lose a fair amount of case compression due to not trapping the incoming charge on the exhaust side. Idk. This is just my theory. I've been studying these stupid little jugs in my spare time for quite awhile now.

It may work and I kinda hope it does but I'm at the point i'd love to get a Wiseco custom made for these things with wider and slightly longer skirts. If there was enough interest the price wouldnt be too bad I guess. No worse than a Dominant piston for a 372. If not that, maybe Mastermind could get the Asians to cast an 026 jug with the ports where they should be. While theyre at it build it into a quad port jug....A guy can dream can't he.

 

[Brewz]

1.8mm lower.... Ouch, that's more than I expected.

Drf255 did predict an outcome like this but I was hoping on being able to get then on done without needing to cut the piston top or squish band too much.

Sounds like it is going to be difficult to get it done with improved timings, but my intake floor is really high at just under 70* (69.8ish*)

That 1.8mm crown height difference would take the dome on the 026 piston into account.

Knowing the difference between the 246 and an 026 piston with the dome machined off is of interest to me.

@drf255 how much shorter does flat topping the 026 piston make it?

 

[drf256]

@drf255 how much shorter does flat topping the 026 piston make it?

It doesn't at all, that's part of my point. The corner of the interface of the side and the crown stays exactly the same.

You can remove a drop, but then your skirt gets too short. Your crown gets too thin also. After you cut the dome, you wind up with the area midway between the center and the edge being .060 on an OEM or meteor. And that's with cutting the edge off flush.

A trick I've used is to leave a rim around the piston edge. Like .008 of height will give you a 1/16" rim around the piston. I know that the dead space isn't exactly the best for combustion geometry, but I haven't had an issue. Then you get some extra height to stop freeporting. You get extra crown thickness too. I hand sand the cut top to achieve my desired squish.

You keep thinking that a shorter piston compression height (distance from pin center to crown) is gonna allow a lower exhaust port. It's gonna do nothing but make you need to overly thin the base to the point where the jug is useless. And your ex roof will be exactly the same.

Picture a degree wheel on your 026. You remove your gasket and your squish is, let's say 0 for illustrative purposes.

So you rotate your stock piston motor to 98* and you see your crown (not 95 because you dropped the jug the thickness of the gasket).

No put a piston with a lower crown height in its place at the same degree reading. You're gonna have to turn the engine closer to TDC to seal the roof. If you're lucky, you'll be at 85 with a .060 shorter height. Drop the jug .060, you're back to 0 squish. You rotate the wheel, you should be back at the same degree, 98*.

Now, there is a chance that I'm wrong here, because crank angle changes piston speed. The piston speed looks like a sine wave. There's nearly no movement at TDC or BDC for a few degrees. The speed is the fastest near 90*, the stroke/cylinder center. But 100* is 100*, so unless I'm seeing it wrong, the piston crown at the same squish with no port mods should be exactly the same.

Sorry for the winded response and this is not meant to be insulting in any way. I hope it's not taken that way.

Like others, I've been searching for a flat top 026 alternative to no avail. I'd like to try an 028 piston. I'd still have to cut the dome off, but maybe it's got longer skirts. I'm not sure.

I like the outside the box thinking on this one Brewz. I'm excited to see your results and I certainly hope it works. I really don't care if I'm wrong or right. I'd rather be proven wrong here and find out we have a suitable alternative to use.

Please post results. I will post some pics when I get my piston.

 

[Brewz]

Oh no offense taken..... You wont ever get that from me mate!
I NEVER believe I know best and will always listen and take on advice from others. Its the only way to learn!

I am 100% hearing you now.
I was taking 026 compression height as pin to piston top, not the outer ridge where the squish is.
You lean something every day

Paper weight it will be. I'm not cutting my one and only jug on a flawed plan experiment.
I guess the 066 can handle a lot more cutting from the base to drop the jug compared to the little 026 jug, something I didn't account for.

Well I guess I will just look into the usual machining to bump performance.

Have you tried machine the piston top at less of a domed angle to get it closer to flat but keep some meat on the top?

 

[drf256]

Randy has. Can't get exhaust as low.

I ordered an 028 piston. I will post some measurements when I have all 3 pistons, the 246/026/028.

 

[Brewz]

It looks like there are a lot of people tinkering with this in their own workshops.

Perhaps pulling everyone together, collecting thoughts and ideas in one place, we will come up with a solution.

Surely we cant let a bit of aluminum beat us.

If you cut the top of the jug to the same angle as you cut the piston top, you should be able to get the same figures shouldn't you.
It would just be a dome within a dome as opposed to a flat on a flat

 

[paragonbuilder]

I'm enjoying the discussion! Carry on!!!


Sent from my iPhone using Tapatalk

 

[czar800]

Al, could you not add material to the piston by welding? I'm thinking that you could add To the skirt as needed? I'm sure this would not be an easy process and it would take some machine after.

 

[drf256]

Al, could you not add material to the piston by welding? I'm thinking that you could add To the skirt as needed? I'm sure this would not be an easy process and it would take some machine after.

A lot of Metallurgy involved. A lot of machining too. Aluminum is hardened before use. Welding makes it soft again.

I thought about building up the entire crown. That may work.

 

[David Young]

The 028 piston has some promise. It's .2mm shorter pin to crown.

But is 4.5 mm longer overall. You would probably have to trim some off because the skirt may hit the crank.

Brian Wright used a 50 piston. Machine off the top trimmed the skirt at milled windows. The saw actually came undone some time later but was a race saw pushed pretty hard.

 

[Brewz]

The other 44mm piston of note is the Dolmar 111

The Meteor catalogue lists them as having identical specs so I wonder why they list them as different pistons?





Curious!

 

[Brewz]

I wonder if the rings are in the same position height wise on the 028 piston?

They look lower and further apart in the picture, but it could just be the picture