Chainsaw Repair

This is a continuation of a build of a 42cc Poulan clamshell that I documented over at AS ( ). 

The cylinder was cut down by 0.040" and the crank/bearing pockets relocated, and I cut a pop-up on the piston.  The exhaust and intake ports are modified, with the the exhaust at 58% of the bore and the intake at 56%.  The port timing is at (duration):
E = 150
I = 165
T= 106

However, that was not what I found interesting.  I have wondered about the design of this engine since I first opened one up, specifically the very wide, flat crankshaft counterweights.  They are much wider than they need to be, and there is no attempt to round them off like I would have expected.  It always seemed intentional but I didn't know why.  After making a cut away engine out of a scrap cylinder, it occurred to me that the counterweights should push a high pressure wave in front of them.  Given the clamshell design, the transfers extend all the way down to the bearings and have a hook shaped profile, which I figure should partially catch some of this pressure as the transfers are opening.  So I decided to try to enhance that effect. 

I designed and 3D printed some inserts for the lower cap to more closely match the shape of the counterweights.  I had to modify the shape of the crank to remove the square shoulders at the pin.  The intent was to try to block some of the air pushed by the leading edge of the counterweights, creating a high pressure at the base of the transfers as they open.  Basically, an inefficient and weak case blower helping to force air up the transfers.  The thin con rod and narrow slot help some too. 

Whether all of this actually works is hard to know for sure, but the saw ran like crazy.  It fast became my favorite, easily pulling a 19" bar with lo pro in anything.  However, the inserts failed twice, and clearly the 3D printed PETG material just isn't appropriate for the temperatures in an engine.  The last attempt held out for maybe 8 tanks, including running the heck out of it on 90deg+ days, but I think the heat soak killed it.  The attached picture shows the remaining intact insert after the last failure - it shows that the leading edge was pushed back and flattened out.  There is no sign of mechanical impact, rather that there was considerable air pressure pushing on that edge.  The trailing edge is fine. 

So that is where the project has been at for a bit while I decided what to do. 

That is some very nice work Chris.    I am curious as to how the JB Weld will hold up.   

On the Poulan Clamshell I did I added some finger ports at an angle going into the transfer ports.   I pointed them in such a way that when the crank was turning it would push the fuel mixture into those ports.   I have no way of knowing what affect it had because I did all my mods at one time.   But I am of the same mind as you that the fuel mixture must be turning in the same direction as the crank.

excellent job Chriss did you ever toy with the idea of a full circle crank?

Chris, nice work with the stuffers. I think the low speed (fps) of the outer portion of these small diameter crank bells limits the ability to create much pressure.  There a lot of info on the sled forums about this, turbo vane cranks, etc.  One study suggested a theoretical  2% pressure rise  from a 5"diameter Crank bell at 8500rpm
take into account losses and you may be back to ground zero?

Chris, nice work with the stuffers. I think the low speed (fps) of the outer portion of these small diameter crank bells limits the ability to create much pressure.  There a lot of info on the sled forums about this, turbo vane cranks, etc.  One study suggested a theoretical  2% pressure rise  from a 5"diameter Crank bell at 8500rpm
take into account losses and you may be back to ground zero?

Thank you for giving it a name and some direction I can start to search under - I figured it must be a known thing but had not found any references.  I agree the small diameter is a limitation, but I'm really only hoping for a modest effect.  Do you know if anyone tried reducing the clearance to the crank weights?

Also I know there isn't any practical way for me to measure if it's effective either - at least nothing I'd take the time to do.  I do know the saw ran very well, and I would not have taken it apart again if the previous parts hadn't failed.

Iv'e used JB weld years ago to reshape intake ports in motorcycle heads , it has the same expansion rate as aluminum but surface preparation for it to adhere is the key  or weld in wedges to anchor the epoxy  to keep it place  . Hopefully you hard work pays off.

I scarred the surface with a Dremel grinder bit and sandpaper, and cleaned it with brake clean.  I'm not too worried about the fillers in the bottom cap, as they have pins and some mechanical restraint in addition to the epoxy adhesion. 

The ones up in the cylinder are the bigger risk, as the jug is plated and that is a harder, smoother surface even down at the lower edge, plus there isn't much mechanical support.  I almost decided to skip those, but curiosity won out.  Still, I did prep it as best I could.