Snowblower Pull Cord Snaps Back

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.

Flywheel

The shear key is made from alloy and if called upon it will break and allow flywheel crankshaft decoupling.

Diagnosing

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, 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.

You should note, a flywheel nut should be torqued to specification and that requires a torque wrench. You’ll find one here on the “Small engine repair tools”.

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.

Blower housing

  • Remove the Blower housing/pull start assembly
  • Remove spark plug

  • Remove the plug wire
  • Impact gun

  • An impact wrench works best. Check out the Amazon link below for decent Impact wrenches at fair prices.
  • Amazon Impact Wrench
    Piston stop tool

  • If you are not using an impact tool, fit the piston stop tool in the spark plug hole. Now rotate the crankshaft counterclockwise until hits the stop
  • Remove flywheel nut

  • Remove the nut, washer, and pull start receiver
  • Shearkey

  • The keyway and shear key is now visible. If the flywheel and crankshaft keyways don’t align, your shear key has indeed broken. This shear key is good.
  • 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

    coil fasteners

  • Remove the armature fasteners and set armature (coil) aside. Turn the flywheel clockwise until it hits the piston stop
  • Flywheel puller

  • Fit the puller tool (some models will require tapping the flywheel first). That’s covered in “Fitting shear key video”
  • Using piston tool, ratchet and socket tighten the flywheel puller tool.
  • Remove flywheel

  • Tighten a little and using a hammer tap on the top of the tool (repeat until flywheel comes loose) With the flywheel free, remove the pullers
  • Shear key

  • Remove the remnants of the broken shear key from both the flywheel and the crankshaft keyways
  • Fitting & Reassembly

    New shear key

  • New Shear key
  • Shear key

  • Refit the flywheel, align keyways and fit a new key
  • Flywheel nut

  • Fit pull-start receiver washer and nut
  • Torque flywheel nut

  • Tighten nut to spec using a torque wrench (usually about 80Nm) (pic shows ring gear locked with lever). Check out the video covering torque wrench use.
  • Remove plug

  • Remove the piston stop tool (if used)
  • Fitting Armature

    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 mechanics 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:

    Coil and feeler gauge

  • Fit the armature in place, but don’t tighten fasteners just ye. Have your feeler gauge to hand.
  • Feeler gauge coil fitting

  • Place the feeler gauge between the flywheel and the armature (check spec). Alternatively use a business card (hack). Push the armature towards the flywheel while tightening the fastener, now remove the feeler gauge.
  • Now go ahead and rebuild in reverse order and get out there and clear that driveway! Good Job!