Reducing Squish on a Poulan Clamshell

[Chris-PA]

I have a nice running ported 42cc Poulan which was the subject of this thread: http://www.arboristsite.com/community/threads/yet-another-42cc-poulan-porting-thread.262612/ However, the squish clearance is large on these engines, and I wondered if I could reduce it. I believe it has been done before, mentioned in this thread: http://www.arboristsite.com/community/threads/wildthing-races-at-gtgs.140451/ but I did not know how. The problem is that the crankshaft must be repositioned in the cylinder:



I got to looking at this design. Unlike some other clamshells these engines are not machined other than the bore and the spark plug hole. The bearing pockets and bottom surface are as-cast (they are actually very nice castings):

Further, the bearings have a pretty thick rubber outer boot that surrounds them and forms the crank seal:


That means these engines can tolerate some slop in the location of the bearing pockets, and that with some sealant it really should not be hard to grind out those pockets - I can get that close by hand. I considered filing the cylinder lower surface with a hoof rasp (OK, a good flat file), but decided to turn it on a lathe instead.

That presented a problem, because these engines are not turned at the factory and so they do not have a boss on the top for a tail support during turning:

There's just a spark plug hole. I wonder if they have a better plug location because of that?

 

[treesmith]

How about extending the rod? Is that an option?

 

[Chris-PA]

I decided to try printing a mandrel on the 3D printer, as I did not want to have to put much pressure up into the cylinder while it was on the lathe in the absence of a good boss for the tail support. This is what I came up with:

It was intended to lock into the bearing pockets for rotation. I decided to print it in ABS, which we had not done much of, since I figured I would need to turn the outer diameter (in the end I didn't have to), and the PET plastic is awfully soft. However, the ABS has a problem in that it shrinks and curls as it cools. I had to try a twice, and a few other experiments too. This took 9:15 to print, and I had to add a supporting structure. Here are some shots of the printing process:


Bottom view showing the support structure:

Here you can see that the outer wings curled - enough that the support structures pulled off the tapered lower portion:

I decided to try it anyway. After cutting away the support, here is how it fit:


I designed it so I could run a 7/16" aluminum hex rod through the center. I added a divot in the end for centering on the lathe. Oh, the shape of the end shaft was so that I would not have to use a lot of pressure on the jaws to keep it from slipping.


I epoxied the rod in with JB Quik

 

[old-cat]

Cut or grind the flat mating surface of the cylinder where the pan fits. Take off the amount of squish reduction you want. Then you have to figure how to mount it in a lathe or mill to cut the bearing bore.

 

[fordf150]

cut your base to give the desired squish and chuck the cylinder in a mill and cut your bearing bore deeper into the cylinder by the same amount as you removed from squish.

 

[Chris-PA]

I had a scrap jug from an earlier project and decided to try it first. Here is the lathe setup:

I need to grind out that wood block next time to better clear the plug boss. Here is the result:

I took measurements on each corner before I cut so I could see how even I got it. I need to spend a little more time indexing it for the real one, but it's pretty good.

That was pretty easy and the tool worked well. Now I had to deal with the bearing pockets, which I intended to cut with a sanding drum on a Dremel. But first I figured I'd need reference tool so I could see how I was doing. I measured the bearing pocket at a 36mm diameter, so I printed a tool at 37mm with thick side walls and turned it down:

I added some grooves so I could tell where the middle is. Here is how it fits in the factory castings:

So I tried it today, and it could not have been easier. Here is one side ground out (notice there is very little material to remove):
Here's how the reference tool fits (I think I'd go just a little further on the real thing:

 

[fordf150]

old-cat beat me to it but i have been thinking about this for awhile since nobody ever talks about doing these motors.

 

[fordf150]

i think at one point a boring bar was mentioned for cutting the bearing pockets deeper

 

[Chris-PA]

I need to wait now until my new piston gets here - the intake will be too low if I cut the one I have. And of course I need to disassemble the engine and measure the squish with the new piston before cutting. So there will be a bit of a delay!

 

[HuskStihl]

I decided to try printing a mandrel on the 3D printer, as I did not want to have to put much pressure up into the cylinder while it was on the lathe in the absence of a good boss for the tail support. This is what I came up with:
View attachment 403561
It was intended to lock into the bearing pockets for rotation. I decided to print it in ABS, which we had not done much of, since I figured I would need to turn the outer diameter (in the end I didn't have to), and the PET plastic is awfully soft. However, the ABS has a problem in that it shrinks and curls as it cools. I had to try a twice, and a few other experiments too. This took 9:15 to print, and I had to add a supporting structure. Here are some shots of the printing process:
View attachment 403562
View attachment 403563
Bottom view showing the support structure:
View attachment 403564
Here you can see that the outer wings curled - enough that the support structures pull off the tapered lower portion:
View attachment 403565
I decided to try it anyway. After cutting away the support, here is how it fit:
View attachment 403566
View attachment 403567
I designed it so I could run a 7/16" aluminum hex rod through the center. I added a divot in the end for centering on the lathe. Oh, the shape of the end shaft was so that I would not have to use a lot of pressure on the jaws to keep it from slipping.
View attachment 403568
View attachment 403569
I epoxied the rod in with JB Quik

That's super-cool. How about a pop-up?

 

[Chris-PA]

How about extending the rod? Is that an option?

Cut or grind the flat mating surface of the cylinder where the pan fits. Take off the amount of squish reduction you want. Then you have to figure how to mount it in a lathe or mill to cut the bearing bore.

cut your base to give the desired squish and chuck the cylinder in a mill and cut your bearing bore deeper into the cylinder by the same amount as you removed from squish.

i think at one point a boring bar was mentioned for cutting the bearing pockets deeper

I too have spent many hours thinking about how to do this, but I think it will turn out to be much easier than all of that. Truthfully, I think I could do it with a flat file and a sanding drum and a bit of careful measurement.

 

[Chris-PA]

That's super-cool. How about a pop-up?

After Treemonkey's latest thread I almost decided to make a tool to cut the squish band and eliminate the small step there. However, there is not much meat on the casting or clearance for the counterweights, and I decided I didn't feel like pushing it.

 

[RedneckChainsawRepair]

3000fps has done something like this a few years ago to tighten up his poulan clamshell. Just using good ole redneck ways.

 

[Chris-PA]

3000fps has done something like this a few years ago to tighten up his poulan clamshell. Just using good ole redneck ways.

Yeah, as I said I have no doubt a file, a sanding drum and some careful measurements would do it. But it's Marbruary and things get dull you know?

 

[RedneckChainsawRepair]

Yeah, as I said I have no doubt a file, a sanding drum and some careful measurements would do it. But it's Marbruary and things get dull you know?

I take it you didnt read the 6 page thread on the redneck way elsewhere. What file are you talking about?

 

[Chris-PA]

I take it you didnt read the 6 page thread on the redneck way elsewhere. What file are you talking about?

You lost me - was there a link I missed?

 

[RedneckChainsawRepair]

You lost me - was there a link I missed?

Yep big time . But this place isnt friendly to other sites with info not here. So no link you have to use google.

Love the 3D stuff by the way.

 

[Chris-PA]

Yep big time . But this place isnt friendly to other sites with info not here. So no link you have to use google.

Love the 3D stuff by the way.

Oh, ok - Depending on which site it is I'll see if I can find it. If it's on the other than I won't.

 

[drf255]

Great job Chris.

Very interested in how it works out.

I got a ton of 4047 alloy here. If you want a small dome welded on that you can turn down, just say the word.

 

[Chris-PA]

Well the piston should arrive tomorrow and I'd like to disassemble the saw so I could get moving on this project. But not in an unheated barn at 10deg and blowing (assuming it actually gets that warm).