Converting Stihl 015L Chainsaw from 1/4″ to 3/8″ Pitch

I received a non-working Stihl 015L Chainsaw the other day to repair. I disassembled and took apart the carburetor where years of neglect and ethanol fuel had taken its toll. After cleaning everything out and a new set of gaskets I got it started and went to town on a dead tree in my backyard. After I quickly realized the tree was starting to smoke and I wasn’t making much progress through the branches. After examining the chain I came to conclusion that… the chain was as dull as a butter knife. I also noticed that the chain would rotate around the bar even when the motor was idling. The service manual said to adjust the idle throttle if this was happening, but even when the motor was about to die it was idling so slow the chain was still turning.

Being quite the novice at sharpening chainsaws I decided to get a new chain instead. Looking online I found a few forums talking about upgrading the 015L from 1/4″ to 3/8″ pitch.

First I would need a new sprocket and needle bearing to match.
I would also need a new chain and bar to run the new setup. I read that a 1/4″ pitch bar wouldn’t work well with a 3/8″ pitch chain even though they were both .050G due to the shape of the nose.

Unfortunately since the saw hasn’t been produced since the 80’s some of the parts were hard to come by. Mainly the oil pump gear that connects the clutch to oil pump, which I couldn’t find anywhere. My local Stihl dealer told me the part wasn’t available for purchase. I found a seller on eBay that manufactures their own replica part, for over $40 a piece. “There has to be a cheaper way for penny pinching Matt!” I thought to myself. After a few minutes of googling I came across this, a STL to 3D print a copy of the gear from the comfort of my own home! Since the gear doesn’t receive and real stress when the oil pump is operating correctly printing this piece out of ABS with 60% infill would be able to withstand the forces applied from the clutch.

The difference from the old and new part was the pattern the clutch uses to drive the oil pump. The sprocket that moves the chain is also used to power the oil pump gear, so if the pitch of the chain changes the positioning of the sprocket had to change as well.

1/4″ vs 3/8″ pitch design

Hot off the print bed! It took me 3 tries with different speeds to find an acceptable quality part. I ended up printing around 20 mm/s, 60% infill, 0.1mm layers, and 0.6mm sidewalls with a .6mm extruder nozzle.

I did have to ream the inside out a little bit to get it to fit on the shaft. I tried to mimic the amount of friction the existing part had.

An image of how the new part interacts with the oil pump gear.

Once the cover was back on things were looking good!

The sprocket and needle bearing were then installed. Fit the printed part like a glove.

side by side the differences between the sprockets becomes more visible

I noticed when I installed the new sprocket that the old one had significant amount of clutch foot material on the outside of it. The clutch is designed to disengage the chain when the engine speed is slow so the saw can be handled easier. Once throttle is applied to the motor the clutch engages and the chain starts to turn. Since there is friction between the clutch and the sprocket the foot has padding on it to prevent metal on metal contact. Since there was a bunch of material built up I thought that might be why the chain was turning while the motor was idling. After installing everything later on I realized my printed part was not deep enough for the sprocket, which was making the sprocket push against the clutch washer, negating the clutch completely and making the chain move while idling. I removed the part and filed out some of the material where the sprocket meets the gear to make it sit flush. After I finished that I could freely spin the flywheel without engaging the chain.

The new chain on top, old on bottom.

Bar size comparison:

Notice the new bar is a bit taller at the end.

Sprocket size difference at the end of the bar to fit the 3/8″ size chain.

The new chain installed around the new sprocket.

I also rewound the starter pulley and spring. Before the handle and rope would hang over the side of the chainsaw when running which was mighty annoying and pretty stupid looking.

I will be checking periodically to see how well the ABS printed part is holding up. I’ve been looking into printing with nylon filament. If the ABS part is not able to withstand the forces put upon it I will recreate it using nylon instead.

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