Poulan 3400,3700,3800, 4000 stroked crankshaft project. Part 1

[dieselfitter]

I have taken a liking to the Poulan model 3400,3700, 3800, 4000 family of saws. They get my vote as the best saw for the money.

I thought it would be fun to experiment with them to get a little more performance. After cutting base and squish on one. I thought it would be fun to increase the stroke for a little more displacement.

The crank was pretty true before I started screwing with it, .0005" runout.

They way they put this crank together, you'd think they don't want it taken apart.

 

[dieselfitter]

I'm going to do an offset crankpin. I have a grind fixture to grind the offset crankpin.

I used a low/reverse servo pin from a 727/618 trans as the new crank pin because it was what I had lying around. I needed to take down the diameter from .614" to .567". The O.D. of the original crank pin is .567". I need to maintain that dimension for the lower rod bearing.


I'm at .0001" over the original, close enough.
Next is to grind the pin offset on each side of the bearing surface down to .474". This will give .093" additional stroke.

 

[huskihl]

I have taken a liking to the Poulan model 3400,3700, 3800, 4000 family of saws. They get my vote as the best saw for the money.
View attachment 43763
I thought it would be fun to experiment with them to get a little more performance. After cutting base and squish on one. I thought it would be fun to increase the stroke for a little more displacement.

The crank was pretty true before I started screwing with it, .0005" runout.
View attachment 43759
They way they put this crank together, you'd think they don't want it taken apart.
View attachment 43765 View attachment 43766

Interesting. We don't get many threads about pressing cranks apart

 

[dieselfitter]

I have the spin fixture set at .1865" offset. I'm going to grind the area of the new crankpin that will press into the crankshaft.

The max diameter this can be is .474". I will grind it to about .472". I have found some inner needle bearing races with that I.D. I will use bearing races as bushings to fit between the new crankpin and the crankshaft halves.

The hole in the crank halves will stay at .567". The original crankpin diameter is .567". The new crankpin will be about .474" where it presses into the crank half. The difference between the .567" hole and .474" pin will be made up with concentric bushings. The bushings will be needle bearing races machined to fit.
For those wondering, I'm planning on using the original connecting rod. I wish I had a bunch of old chainsaw guts lying around to mix and match parts. I might have considered a different rod, piston and cylinder....

I ground the new crankpin down where it will press into the crankshaft halves. The material outside of the two ground recess's will be cut off and discarded.

I ground the bearing races that I am using as bushings. I had to grind the OD to .471" to match the the OD of the original crankpin.

 

[dieselfitter]

Then I pressed them into the crank halves.

 

[dieselfitter]

I pressed the rest together.


The moment of truth. Run out is .0002"

 

[dieselfitter]

Forgot a pic
My goal was to be under .005" The unmolested 3400 crankshaft I have runs out at .0006". Scott/Treemonkey had one done that ran out .020". He was not happy with it.
More later...

 

[dieselfitter]

Stock stroke is about 1.319". I ended up adding .094" for a total of 1.413".
Using a 3800 piston and cylinder, the displacement will now be 64cc With a 4000 piston and cylinder(1.9375" bore) it will be 68cc.


The case had to relieved to make room for the piston.

I modified a mandrel I had made to cut the base. I can now cut squish with it also.

I pulled apart my red Craftsman and put the stroked crankshaft in it with the 3800 top end. After cutting the squish on the 3800 jug, the piston barely touched without a gasket. I put in a .020" gasket and ran it today. It runs ok. It seemed to have a little more torque but that is a subjective observation.Ok so here is a video.
This is a Craftsman branded Poulan 3800, crankshaft has been stroked 3/32", squish cut, base is stock timing advanced and muffler modded. Nothing revolutionary, I'm guessing it is on par with a stock 4000.
<iframe width="560" height="315" src="https://www.youtube.com/embed/XAfu_wVwa0E" frameborder="0" allowfullscreen></iframe>
Not awe inspiring. I would like to get a ported 4000 cylinder on it.

I'm not done with experimenting with the Poulan model 3400, 3700, 3800, 4000 family of saws yet. I'm going to try making a 5 piece crankshaft. That will allow me to change the stroke quite a bit more.

 

[Dub11]

oh I've been waiting fore this!!!!

 

[Red97]

Very nice.

 

[Dub11]

So are all crankshafts put together lile this?

 

[Red97]

So are all crankshafts put together lile this?

Most newer ones are. Lots of old saws were 1 piece with capped rods and needle rollers.

 

[Sty57]

This is an awesome build Tim.

 

[LBracing]

I have build some stroker crank this way also , works great , I welded mine for safety reasons tho . Never had one fail but I wouldn't put it in one of my full blown racesaw. Nice work !

 

[Redfin]

Great read. Thank you.

 

[dieselfitter]

I'll be putting a 4000 cylinder on it and Adam(Jonseredracket) will be porting. I will post a video when done. This thread is part one of two threads. Part 2 will be the 5 piece crankshaft with 41.1mm stroke for the same saw. I am not ready to start that thread.

 

[Mycamaro_68]

Awesome thread @dieselfitter! Reading this reminded me of Arrowhead's "ultimate sleeper saw" on AS. This would be a good way to get him back up to 72cc. I looked him up on AS and it looks like he hasn't been active for a while, but I bet he would be interested in this thread.

 

[dieselfitter]

Awesome thread @dieselfitter! Reading this reminded me of Arrowhead's "ultimate sleeper saw" on AS. This would be a good way to get him back up to 72cc. I looked him up on AS and it looks like he hasn't been active for a while, but I bet he would be interested in this thread.

Thanks,
My next planned project is a stroked Poulan 3450 with a Stihl 380 magnum piston and cylinder. 36mm stroke x 52mm bore,82cc. Plan is subject to revision. Right now I need to finish the 5 piece 41.1 mm stroke version of a Poulan 3400.

 

[Mycamaro_68]

H

Thanks,
My next planned project is a stroked Poulan 3450 with a Stihl 380 magnum piston and cylinder. 36mm stroke x 52mm bore,82cc. Plan is subject to revision. Right now I need to finish the 5 piece 41.1 mm stroke version of a Poulan 3400.

Have you read Arrowheads thread? He put an 038 mag jug/slug on a 2800. Pretty neat build. I have a 2700 and a 272xp top end that I thought about looking into a Poulan/Husqy franken-saw. That 3450 with 82cc would be an awesome little saw!

 

[Mycamaro_68]

Also, how does adding stroke affect the port timing? All of this is very interesting and I try to learn anything I can.