Poulan 380/3750 -- Stihl 039/390 piston

[Benwa]

It all started with a $100 dollar craigslist find. It didn't look abused, so the purchase was made.

Typically, it was filthy by my standards.


but it all cleaned up well.

The saw ran very well and looked great inside (through the exhaust port). That being said, a compression test only yielded 125psi.

 

[Benwa]

With minimal effort the case and tank just pulled right apart.

Maybe the squish of .048" with factory base gasket had something to do with it only pulling 125psi.

I could only image how this saw might run with a little more punch

So, I was on a quest for a reasonable squish number and to potentially figure out a piston option, other than stock, for those who may be in need. There are so many different ways to go about achieving a target result, but here's where I landed.

After searching for 49mm pistons and their specs (not always easy to find) I settled on the piston for the Stihl 039/390. Availability of 10mm wrist pin being a major deciding factor. Meeting two of the requirements, I decided to order one up and spec it out when it got here.

 

[Benwa]

I knew from some of the details I found, being a 2 ring piston, etc. that the pin to crown height was most likely more than the stock piston.

So here are some specs. These measurements may already be common knowledge on here, but I can be computer illiterate at times, so they are new to me.

The following measurements reference wrist pin centerline.
Piston crown height
Stock -- 0.741"/18.84mm
Meteor 039 -- .804"/20.44mm

Skirt length
Stock -- 0.656"/16.66mm
Meteor 039 -- 0.651"/16.55mm

Distance from top of upper ring land to piston crown
Meteor 039 -- .118"/3mm

Width of ring locating pins
Meteor 039 -- 1.250"/31.75mm

*Note*
Stock piston isn't "windowed," but has open sides. The 039 piston is what I would consider a full circle piston although there are voids on the sides at the pin boss.

 

[jk14]

Interesting, you gonna cut the squish band or trim the piston crown? Or maybe a popup?

 

[Pincher]

A thick base gasket?

 

[Benwa]

Interesting, you gonna cut the squish band or trim the piston crown? Or maybe a popup?

A thick base gasket?

You're both on it
I cut a pop up, but it's still too tall. So for now I used a thicker gasket. Cutting the band would be optimal, but I don't have the means right now.

 

[Benwa]

So we have the correct bore size and 10mm wrist pin. Regardless of modifications needed, is it even possible to make this thing work in this saw? Before I get into piston and/or cylinder modifications, there's one caveat. The 039 piston is a two ring piston and therefore has not one ring locating pin that is centered, but two that are offset.

Cylinder port widths
Intake -- .982"/24.95mm -- 51% of bore
Exhaust -- 1.020"/25.90mm -- 53% of bore

Width of ring locating pins
1.250"/31.75mm

So we are good, well very close, but ring ends will clear the ports. Oh wait, the transfers........
Using the witness lines in the cylinder from the stock piston, it seems that the piston skirt is RIGHT at the edge of the transfer ports on the intake side.

So I measured the

Stock piston skirt width
1.330"/33.80mm

I know this is a "rough" measurement, but in my eyes this only leaves .040"/1.01mm between the locating pin/ring ends and the transfer port. To close for comfort in my eyes.


So.....can the piston be installed backwards? I have nothing against it so let's see.....

 

[Benwa]

These cases were obviously used for many applications. The cylinder base is not relieved the same as it is in the case for the lower transfers. So using a gasket that was case matched, I marked the cylinder base. The piston actually matches the lower transfers much better if reversed........ bonus!

Correct orientation



Backwards



Now we know it fits......well, now we know we can MAKE it fit.

 

[Benwa]

With only having .118"/3mm of material above the top ring land, I can only take .018" off and still keep the .100" I like to adhere to. Well, being a personal saw, I fudged it went for .020" overshot and took off .022" leaving me with .096" above the land. I'm okay with that on my own saw.
Besides that, I got rid of a little extra weight haha.

Stock weight with wrist pin and ring


Meteor 039 with wrist pin and rings


Meteor 039 After popup
Decided I'll only be running 1 ring for now, more about that later.

 

[Benwa]

Well, the piston is done, time for the cylinder.

Factory squish with base gasket .048"

Difference in crown height between stock piston and meteor 039 -- .063" taller.

So the 039 piston is .015" taller than the cylinder squish band......... well this won't work. So.......

Take .022" off the top of the 039 piston and it leaves us with .007" squish.

If I had the means, I would cut the squish band and be done with it, but I don't. So I added a gasket the correct thickness to make up the difference and ended up at .020" squish.

Factory port timing numbers
Exhaust -- 100°/160 duration
Intake -- 78°/156 duration
Transfers primary 124° secondary 126°
Blowdown -- 24°

It's no wonder these saws run so strong in factory trim. The port shapes are on point and congruent with the timing numbers.

 

[Benwa]

So I decided that it only made sense (especially with full circle piston) to case match the lower transfer area in the cylinder base. Taking other saws with this design into consideration and talking with a few people, including @Kensie1988 who has recently been into a 3750 himself, I'm confident in this decision.




After everything was bolted back down I ended up with

New timing numbers
Exhaust -- 102°/156 duration
Intake -- 77°/154 duration
Transfers -- 126°
Blowdown -- 24°

I couldn't widen the exhaust any because of the locating pins and for this reason I left the intake width alone as well. Besides the intake already had a great profile to it. I ended up taking the ports back to their original timing numbers. I would usually go a little further on intake and transfers, but this allows me wiggle room for when the band is cut and the jug is dropped back down.


I wish all saws had an intake port like this


I plan on going back in and doing a touch more port work once I have the means to cut the squish band and go back to factory base gasket.

 

[Gaston]

Hello all. My name is Stephen. This is my first official post. I hope I am not derailing this thread. I had the same idea about 1-2 years ago. I had a 039 Cross pop up piston and a bunch of poulan parts. Used a 3500/3600 cylinder and the 039 pop up piston. I don’t have access to a lathe so I sanded the squish and used a thicker base gasket. But I decided to modify the piston instead of the cylinder, just to make sure it worked. I can get pic tomorrow if desired. It is not perfect by any means but runs pretty good

 

[Benwa]

Hello all. My name is Stephen. This is my first official post. I hope I am not derailing this thread. I had the same idea about 1-2 years ago. I had a 039 Cross pop up piston and a bunch of poulan parts. Used a 3500/3600 cylinder and the 039 pop up piston. I don’t have access to a lathe so I sanded the squish and used a thicker base gasket. But I decided to modify the piston instead of the cylinder, just to make sure it worked. I can get pic tomorrow if desired. It is not perfect by any means but runs pretty good

Not derailing at all! We all learn from sharing and love seeing others projects. Especially since it's on the same topic! I'm in........Post 'em up!

 

[Benwa]

Rings still had assembly oil on them so I'm hoping it'll land in the 185ish range once it's all broke in.


I had already modified the muffler, but still had the stock outlet on one side.

 

[Benwa]

So I decided to match the side that I already modified. I use the @Red97 deflectors. My favorite deflector to use hands down. Trim them any shape you want. A little heat from a torch and you can manipulate them around any irregularities you need to on the muffler.


I have no idea what this threaded boss is for on the saws, but I'm guessing a different application on some other model.


I trimmed it off so I could fit a matching deflector on the clutch side where the factory outlet was. Here's the deflector all fitted up and brazed on.


Factory spark screen is still inside.

 

[Benwa]

I also bumped the ignition timing
about 8°

After a few heat cycles I'll put it in wood and see how it does.

Here it is bone stock

And with a dual port muffler

I'll throw an updated video up of it running with the current modifications once I get it broke in a little.

 

[qurotro]

Interesting..

 

[Benwa]

One just can't leave well enough alone, so I ordered a 24" Tsumura lightweight bar for it.

I modified the Husqvarna K095 mount to match the Poulan K041 mount. Both bars are a 8.2mm slot, but the K041 oils through the bar adjuster hole instead of its own oiler hole like the K095.

So you have to elongate the bar adjuster hole toward the rail until you break through the bar groove


Much better

 

[smokey7]

Very nice job. But I fear your tune in the last vid is more a can't racing tune than a working tune in large wood. I think you would be ok in small wood with a fast tune like that tho.

 

[Benwa]

Very nice job. But I fear your tune in the last vid is more a can't racing tune than a working tune in large wood. I think you would be ok in small wood with a fast tune like that tho.

You're absolutely 100% correct. Factory max rpm is 13.8k.........supposedly. I don't buy it, especially with the factory stuffed up muffler. Audibly, it was 4 stroking more in person than what the video let on, albeit not heavily. After running it a bit more that day I came to the same conclusion you did. It was leaner than I like to run things. So, since then, I have had it tuned back to about 13.4k hot out of the cut (in bar length wood) with a nice heavy 4 stroke. I usually run my stuff on the safer side of things. Thanks for the input, it's appreciated and good ear!