By: Author John Cunningham. Published: 2021/06/15 at 4:19 pm
Ouch!! Many of us have experienced the bite of a mistimed engine, and it stings… a lot!. While it may seem like the engine is out to get you, it’s just the engine’s way of letting you know all is not as it should be. A few minutes from now, you’ll be wise!
Snowblower pull cord commonly snaps back because the flywheel shear key has broken. The shear key helps maintain the flywheel crankshaft relationship. When the shear key breaks, the engine timing is off, which causes the pull cord to snap back sharply. Replacing entails removing the flywheel and installing a new key.
In this post, you’ll learn why your snowblower pull cord snaps back. You’ll learn how to replace the shear key and why it breaks in the first place.
What Is A Shear Key?
A shear key is a small block of alloy used to locate the flywheel on the crankshaft. Both the crankshaft and the flywheel sport a machined keyway; the shear key fits neatly into the keyway and aligns both components.
It’s important that the crankshaft and flywheel are aligned correctly and coupled. Any variation in their alignment will cause the timing of the engine to be off. The shear key has another important job, though.
It sacrifices itself by shearing; it does this to protect the crankshaft.
The shear key is made from alloy, and if called upon, it will break and allow flywheel crankshaft decoupling.
A pull cord that snaps back is a sign of a broken shear key. But to know for sure, we’ll need to remove the pull starter assembly and remove the flywheel nut. Doing so allows us to view the keyway, and a misaligned keyway is easy to spot.
Common symptoms include:
- Pull cord hard to pull
- Pull cord snaps back
- No start engine
- Broken pull cord handle
- Oil leaks from engine
Checking Snowblower Shear key
In the workshop, I use an impact wrench which makes flywheel nut removal easy. If you don’t have one, you’ll need a piston locking tool and old-timers stuffed rope into the cylinder to lock the crank, but I wouldn’t advise it.
You can check out the piston locking tool I recommend here “Snowblower maintenance tools page.”
The following steps assume you don’t have an impact wrench; if you have, great! Skip the piston tool steps.
Removing Snowblower Flywheel Nut
Note if you are using an impact wrench, you won’t need the piston stop tool. As an alternative to the piston stop tool, have a helper lock the ring gear with a screwdriver as per the picture shown below in the re-fitting process.
Replacing Snowblower Shear key
Replacing the shear key itself is easy; the hard part is accessing it. The flywheel can be stubborn to remove and will likely require a tool known as a flywheel puller. It’s not an expensive tool, but it is essential.
Prying or hammering on the flywheel is a no-no; it will damage the flywheel and possibly the engine. You can check out the flywheel tool I recommend here on the “Small engine tools page” or check out the Amazon link below.Amazon Lawnmower Flywheel Puller
Here’s the stepped process, starting where we left off above (flywheel nut off). Note your engine may be slightly different and so some steps may differ slightly.
Removing Snowblower Flywheel
Fitting & Reassembly
A special procedure is required to fit the armature, it’s not complex, but it is important. A tool called a feeler gauge is required, and ideally, the specification of the armature air gap should be referenced.
The gauge isn’t essential, and I’ll show you a mechanic’s hack that will do the job, also. However, if you want the gauge, you can find the feeler gauge here on the “Small engine tools page.”
The armature fitting process is as follows:
Now go ahead and rebuild in reverse order and get out there and clear that driveway! Good Job!
- About the Author
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John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.