How to tell if lawn mower crankshaft is bent

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A bent lawn mower crankshaft can cause catastrophic consequences.

The blade that’s spinning at 3000 rpm (possibly more) can, for instance, break off when mowing with a bent crankshaft and cut off one or both your feet.

I will teach you how to tell if lawn mower crankshaft is bent in this guide and elaborate on some of the common remedies for this hitch like straightening a bent crankshaft.

We will first discuss the various ways of telling if this essential mower part is deformed.

How to tell if lawn mower crankshaft is bent

You can either use the known symptoms or visually inspect the mower for signs of disfiguring.

We shall start with the telltale symptoms:

1.      Look out for the typical bent crankshaft lawn mower symptoms.

A sure way of telling if your crankshaft is crooked is to look/listen out for the typical signs.

Here are the bent crankshaft symptoms to be keen on if you suspect that your cutting machine’s crankshaft is bent:

  • Excessive vibrations- if your mower starts to make excessive vibrations/grinding sounds when cutting, the lawn mower shaft may have bent.

It might even produce the loud noises for a couple of seconds then run fine again before repeating the annoying noises at intervals.

  • Transmission issues might occur- another indicator of a bent crankshaft is persistent and strange startup issues.

These are incidental to the bending.

For example, the impact that first made the crankshaft to bend may also have broken the flywheel, messing the ignition timing and the mower may become a problem to start.

2.      Conduct a physical inspection

Another approach that works is to physically inspect the machine.

This is especially helpful where the symptoms are absent or confusing.

Here are the steps:

How to check for a bent crankshaft physically

I recommend any of the following procedures.

Method 1

  • Start by disconnecting the spark plug and turning the mower over.
  • Using a marker, mark one end of your blade (on the cutting deck skirt). You could also clamp a ruler (use duct tape or vise grips) from the edge of the mower’s deck to where the ruler’s end touches the shaft.
  • Next, manually turn the shaft while observing the gap between where the ruler ends and the shaft through one rotation (remember to wear safety gloves).

Your crank is warped if the gap first expands then grows smaller.

Method 2

  • Remove the mower’s spark plug.
  • Now ask a friend to slowly pull the starter cord (hold the blade’s brake handle back). 
  • Your eyes will be attentively fixed on the bolt holding the blade (onto the shaft) as he/she pulls on the rope to spin it.

Any wobble (the bolt tends to move from sideways while the shaft turns) could point to a bent crankshaft.

Method 3

An alternative way involves you visually observing it for noticeable blade wobbling as explained below.

  • Park the mower over a flat surface (where there’s no debris) but leave the engine running (and the blade engaged).
  • Closely observe it for any signs of the blade shaking (the discharge port can make a good observation point).

A wobbly blade can often be due to damage to the blade itself but can also signal a twisted crank.

Method 4

The hi-tech way of verifying if you have a bent shaft is by employing a dial indicator (aka a “clock”).

This is usually positioned on designated points depending on the engine model and is the preferred technique among experts.

What causes a bent crankshaft?

In most cases, the crankshaft bends after hitting immovable obstacles like a large rock, stumps, pipes, sprinklers, and more with the engine still running.

Other potential causes are dropping the crankshaft during installation at the factory or other improper handling practices during maintenance.

How do you fix a bent lawn mower shaft?

As mentioned earlier, a distorted crankcase is a hazard and needs to be addressed immediately.

The most obvious solution is to have the bent crankshaft replaced by an OPE repair shop (they will also mount new crankshaft oil seals and piston rings).

But that will cost a hefty amount of money and you may want to consider cheaper remedies such as straightening a bent crankshaft.

Some service professionals use a lawn mower crankshaft straightening tool for this procedure and it should bring your back your mower to normal functioning and reduce lawn mower vibration immensely.

And it’s not terribly complicated and you can easily undertake the task safely at home (YouTube videos can help you learn how to remove lawn mower blade shaft).

Some owners reportedly use sledgehammers to do the job but we suggest that you consider buying a small engine crankshaft straightener if under a budget (you don’t want to break the crank!).

You should ,however, beware that some manufacturers discourage straightening of bent shafts- whatever the cause- and will cancel your warranty.

In most extreme cases, you might be forced to replace the engine or junk the mower (you can strip it for parts).

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Wrapping it up

A bent crankshaft can trigger a freak accident leading to disasters like the loss of toes and it’s important to discontinue mowing if you are concerned that your cutting machine could have distorted the crank.

To learn more on how to fix a bent lawn mower shaft watch this video below.

You now understand how to tell if lawn mower crankshaft is bent so you should easily confirm your fears.  

Having said that, always walk the planned mowing area and clear obstructions like rocks and tree roots (and put a perimeter wire/boundary where necessary) before commencing your cutting duties.

That can drastically reduce your chances of hitting them and bending the engine crankshaft.

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