How To Tell If A Lawnmower Shaft Is Bent Or Broken

Ian Haynes
by Ian Haynes

Did you hit a rock, a stump, a root, or maybe even a metal pipe while mowing your lawn? If the answer is yes, then chances are that your lawnmower starts vibrating violently and you have no idea how to fix it.

If the crankshaft is vibrating, making harsh noises, or sputtering while starting, it is most likely your shaft is bent or broken.

A bent lawnmower crankshaft can be very dangerous. With speeds of up to 3000-4000 rpm, the blade can break off and slice your feet right off if you aren’t careful about using the machine.

Let’s first figure out how to diagnose a deformed crankshaft.

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How To Tell If A Lawnmower Shaft Is Bent?

You can determine this by either visually inspecting the mower for any signs of deformation, or check to see if your lawnmower is displaying the common symptoms of a bent crankshaft.

Judging By Symptoms

What are the typical symptoms of a bent crankshaft on a lawnmower? Keep an eye (and ear) out for some of these symptoms to check if your crankshaft is bent.

  • Excessive vibrations: If your mower is making grinding sounds and vibrating excessively, your shaft may have been bent. The noise can come on just a few sounds before running fine. Look out for unusually loud sounds coming out of your mower
  • Transmission issues: Like a car, your lawnmower might also have problems running and starting up. These issues are typical with a bent crankshaft. It’s possible that your crankshaft also hits the flywheel, messing up the ignition

Click here to find out how to start a lawnmower with a bad starter.

Physical Inspection

If you can’t determine by checking the common tell-tale symptoms, you can go for a physical inspection. There are different methods for determining this, and this is how you do them:

Method 1
  • Disconnect the spark plug and turn your mover over
  • Mark on one end- the cutting deck skirt. Clamp a ruler from the edge of the deck to the ruler end where it touches the shaft
  • Turn the shaft with your hands and observe the gap where the ruler ends and the shaft through one rotation

If the gap first expands, then contracts, your crankshaft is deformed.

Method 2
  • Remove the spark plug from the mower
  • Ask someone to pull the starter cord
  • Keep your eyes fixed on the bolt that holds the blade and check for any wobbles

If the bolt wobbles or moves from side to side, that means your crankshaft is bent.

However, if you do not have anyone to pull the plug for you, you can try another trick. Park the blade at a distance with the engine running. Check if it wobbles.

Method 3

With recent lawnmowers, you can also use a dial indicator to verify if your crankshaft is bent (if your lawnmower has that feature). This is usually positioned on the designated points depending on the model of the lawnmower.

Causes Of A Bent Crankshaft

The blade tip of the lawnmower whizzes at speeds of up to 200 mph, so it also needs massive amounts of kinetic energy to operate. When your lawnmower encounters a sudden stop due to hitting an immovable object, that energy is dissipated through the blade and shaft, causing it to bend.

The most common cause of a crankshaft deforming is hitting an immovable obstacle while mowing, like a big rock, pipes, sprinklers. After all, mowers are meant to cut grass, not rocks and metal pipes.

Other uncommon causes can be that somebody dropped the shaft while installation or some other accidents or defects during the manufacturing process. Improper handling during maintenance can also be a cause sometimes.

Do you want to detect concealed sprinklers to avoid damaging any accidents with your lawnmower? Click here to find out how.

How To Fix A Bent Lawn Mower Shaft?

If you do not want to pay a mechanic to fix your crankshaft, you can try a little DIY and fix it yourself. Here’s how you will start the straightening procedure, but before that, take a look at the tools you will need:

Tools You Will Need

To get the ball rolling, please have these tools at hand before you start:

  • A socket set
  • Nut drivers- size may vary by lawnmower but generally 5/16”, 3/8”, 7/16” and 1/2 “
  • Screwdriver- To pry open the flywheel
  • A heavy pipe
  • Rubber hammers
  • Wrench
  • Valve spring compressor tool
  • Vise grips

Access The Shaft

Before straightening the shaft, you will have first to locate it for which you will need to:

  • Start on a cool lawnmower and unscrew the oil plug to drain out the oil completely
  • Close the fuel hose and disconnect it. Open the gas cap and drain the fuel out
  • Disconnect the air filter assembly, unhook the engine and throttle cables, and pull out the oil tube
  • Remove the clutch and slide a small piece of wood between the flywheel and housing
  • Loosen the bolt on the flywheel and pry it loose
  • Pull the shear pin out from the crankshaft. Turn the bolt on the blade, pull it off and the bolt and set it aside
  • Disconnect the bolts that hold the deck and engine. Pull out the motor
  • Disconnect the screws holding the valve spring cover
  • Push down on the spring with the spring compressor tool, let it compress, and then carefully let go
  • Repeat on the second valve spring
  • Pull out the camshaft gear from the blade, unscrew the bolts to remove the rod end cap
  • Push down on the connecting rod till the cylinder piston moves up and then turn the crankshaft slowly. Move the crankshaft off the engine

Straighten The Shaft

Once you have located the pole, you can begin straightening it in this way:

  • Place the bent shaft on a hard surface
  • Set a heavy pipe over the bent shaft
  • Hammer the pipe over the shaft a few times and then remove the pipe
  • Check the straightness of the shaft using a level
  • Hammer again until the shaft assumes a straight shape
  • Reverse the disassembly process and put the mower back again

Other Considerations

Attempting to straighten your bent shaft by yourself or even by a third party technician can invalidate your warranty unless a licensed service agent services your mower.

Do not attempt a DIY if you are unsure of how to do it. Get it done by a technician, preferably a licensed service agent.

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Related Questions

What Happens When A Crankshaft Goes Bad?

If your crankshaft position sensor has any issues, it can cause the signal to cut off with the engine running. This can stall the engine. Usually, this is a wiring problem, but a crankshaft position sensor can also be the reason.

How Much Does It Cost To Fix Crankshaft?

The average cost to repair a crankshaft goes from $50 to $100, with labor costs being about $130 to $160. However, crankshaft repair is still cheaper than a total replacement.

Can You Drive With A Bad Crankshaft?

If you have symptoms of a faulty crankshaft, do not drive your mower to damage your engine extensively. Get it repaired before driving it again.

Ian Haynes
Ian Haynes

Ian Haynes is a digital marketing specialist and has successfully written hundreds of home improvement guides. Outside of his work, Ian likes fixing old bikes and exploring Brooklyn with his Labrador.

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