Lawn mower broken crankshaft. Can You Run Mower Without Blade? – don’t do it

Can You Run Mower Without Blade? – don’t do it!

Many people ask me about starting their mower engine without the blade, so much so, I wrote a post explaining all.

Many small engines are designed specifically for mowers and as such require the extra mass of a moving blade to help rotate the crankshaft on the compression stroke past top dead center (TDC). Starting such an engine without the blade may cause personal injury and damage the engine.

In this post, you’ll learn why most mower engines require a blade to run correctly. If you need video help, check out “Replacing mower blade video”.

Most mower engines won’t start without a blade attached.

Small Engine Types

There are two common engine configurations, the Vertical crankshaft, and the Horizontal crankshaft layout. Vertical and Horizontal crankshaft is self-explanatory. The orientation of the crank will naturally lend itself more suitable to one task over another.

Horizontal Engine

It’s not possible to turn a Horizontal shaft engine vertically and simply call it a Vertical engine. These engines have been designed to operate in their particular configuration. For example, most small engine oil systems are splash lube, it relies on splash paddles hitting a puddle of oil at the base of the engine. You can see the problem here. But the differences go deeper than that, as we’ll see shortly.

Horizontal small engines are designed to suit many applications, unlike vertical engines. The Horizontal engine is more common generally. They are best suited to driving power washers, water pumps, cement mixers, tillers, snowblowers, etc. They are not designed specifically for anyone single purpose and as such will run just as well with or without an attachment on the crankshaft.

Vertical Engine

Mower engines on the other hand are a little different. They are known as vertical shaft engines. Vertical shaft engines by their layout are perfectly adapted to sitting on top of a mower deck with a blade fastened to the crankshaft underneath.

Most of these types of engines are designed to have a blade fitted, meaning, the mass of the rotating blade is factored into the overall design of the motor. I say most mowers because some manufacturers employ a feature like Toro’s Spin Stop. They use a separate control lever to engage the blade. This feature allows the engine to run without the blade spinning, useful for propelling the mower across graveled areas.

Higher-end Honda mowers also sport this feature. If your mower has this feature then your engine will comfortably run without a blade attached. Likewise, tractor mower engines will run happily without blades attached, since they aren’t designed to have a blade fitted directly to the crankshaft.

Why Does Mower Need Blade To Start

Before explaining why a mower requires a blade, it’s helpful to know a little about the four strokes of an engine cycle. Once understood, the purpose of sufficient mass to rotate the engine becomes clear.

Stroke 1 Induction – As the piston travels down the cylinder the intake valve opens. Gas and air mixture enter the cylinder.

As the piston nears the bottom of the cylinder the intake valve closes.

Stroke 2 Compression – The piston meets resistance as it travels back up the cylinder, all valves are closed. Gas and air mixture is forced towards the combustion chamber at the top of the cylinder by the piston.

(It’s momentum, helped by the mass of rotating crankshaft, flywheel and blade that powers this stroke).

Stroke 3 Power – As the piston passes the very top of the cylinder travel (TDC), the plug fires and the explosion drives the piston downwards towards the bottom of the cylinder.

This of course also drives the blade which cuts the grass.

Stroke 4 Exhaust – This is the last of the four strokes of a complete cycle. The piston begins to travel back up the cylinder and the exhaust valve opens to allow spent gases to exit through the muffler.

As the piston reaches the top of the cylinder, the exhaust valve closes and the whole cycle begins again.

Mower Engine Needs A Rotating Mass

Flywheel Adds Mass

All small engines (vertical and horizontal) require a rotating mass to drive the piston when it’s not under power. This is especially true on the compression stroke, where the piston meets lots of resistance as it squashes the fuel mix into the combustion chamber.

To help the engine achieve sufficient rotational momentum, it employs weight in the form of a crankshaft, flywheel and of course, the blade is factored when the engine is designed for a mower.

If a moving object has sufficient mass (weight), it will carry momentum. Without the required mass rotating at a sufficient speed (mower blade spins at over 200 mph at the tip), the resistance the piston meets on the compression stroke would simply overcome the rotational force on the crankshaft and the engine loses its momentum. Firing the plug now easily causes the engine to reverse track, aka kickback.

Spinning Blade Adds Mass

Running Mower Without Blade Is Dangerous

Attempting to start a mower without a blade is dangerous, the lack of crankshaft momentum causes the engine to kick back. Kickback occurs when you pull the starter cord to start the engine and the cord is snapped from your hand sharply. The cord often whips you as it snaps back and sometimes breaks the pull handle. This can be a painful experience and is more commonly associated with a broken shear-key.

You can read more about a shear-key here.

Broken Shear-key

Running Mower Without Blade Risks Engine Damage

Running the mower engine without the blade risks damaging the crankshaft or shear-key. Blades aren’t universal, a mower engine is designed to run with a specific blade, by specific I mean not only correct length and width but also mass and lift.

As an engine approaches top dead center, and if it isn’t carrying sufficient momentum. As the plug fires, it causes the crankshaft to rotate in the wrong direction, snapping the cord from your hand as it does so. If you need video help, check out “Fitting mower shear-key video”, which covers the complete process from diagnosis to replacing, or check out a post I wrote here “Replacing a shear-key”.

Hey, I’m John, and I’m a Red Seal Qualified Service Technician with over twenty-five years experience.

I’ve worked on all types of mechanical equipment, from cars to grass machinery, and this site is where I share fluff-free hacks, tips, and insider know-how.

And the best part. it’s free!

How To Tell If A Lawnmower Shaft Is Bent Or Broken

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.

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.

lawn, mower, broken, crankshaft

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
lawn, mower, broken, crankshaft

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.

How to tell if lawn mower crankshaft is bent? Common symptoms with solutions

While using the lawnmower, you will face some problems with this machine. In the previous article, we mentioned fixing plastic gas tanks in the lawn mower.

Today, we will talk about a new issue that is lawn mower crankshaft. If you’ve used a mower, you may have had this problem. And many users don’t know how to fix it.

The crankshaft can cause terrible accidents for you, so you have to learn about it. But how to tell if lawn mower crankshaft is bent? Keep in mind in the article below and we will tell you all about it.

Where does a bent crankshaft come from?

As you know, the blade of your lawnmower always rotates at very high rpm. Some mowers have an rpm of around, but it can reach 3000rpm/around or more. With that speed, when the crankshaft hits an immovable obstacle, it can be bent.

Fix a Bent Lawn Mower Shaft

An immovable obstacle can be a fence, a rock, a sprinkler, or something more. When your lawnmower hits one of them while the engine is still running, 100% it will be bent.

During maintenance and repair, the crankshaft was installed incorrectly as another reason for the bent crankshaft. You can complain to the repair facility or the place where you guarantee the product. I’m sure they will repair it for you or completely replace the crankshaft.

Please pay a little attention to the machine during operation to find out what causes the bent crankshaft. It can help you avoid many bad problems after.

How to tell if the lawn mower crankshaft is bent?

Typical symptoms of a bent lawnmower crankshaft

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Excessive vibrations

The first symptom is excessive vibrations. During operation, the machine produces abnormal vibrations continuously and especially makes loud noises. Those are obvious signs of a bent or broken elbow. These symptoms only occur when the crankshaft part of the machine has been bent or broken.

The transmission

The second symptom is related to the transmission. This is quite an important issue. For a machine to work properly, all the parts of the machine need to work well and smoothly. The damaged crankshaft of the machine will cause collisions with other parts, making the transmission system inoperable. If this happens, you will meet a lot of difficulty in starting and operating the machine.

Physically inspect for a bent crankshaft

A physical inspection of the lawnmower is very important. We need to know the situation of the machine when the symptoms begin to appear before they become too serious.

The GAP between the Crankshaft and the Deck

The first thing that comes to mind when doing a physical inspection of a lawnmower is a broken crankshaft. That is to check the clearance between the crankshaft and the deck. If the clearance changes as you rotate continuously, your crankshaft is bent and needs repair. When performing this method, always remember to disconnect the spark plug first.

Look for Bolt Wobble

With this method, we will first disconnect the spark plug. Then have someone pull the cord to start the lawn mower. Meanwhile, watch the bolt that holds the blade to the crankshaft if any movement or wobbling means the crankshaft is bent and in need of repair.

Use a Dial Indicator

The dial indicator is an instrument with a dial that measures or detects minimal changes in distance and amplification that can be observed by anyone. At the same time, we can also use it to check if the elbow is bent or not. We just need to place the dial indicator at a specific point on the elbow and we will get what we want.

Look for blade wobble

Just like the swinging bolt of the crankshaft is bent, so the swinging saw blade is also a harbinger of the bent of the crankshaft. To see the blade, let the machine start on flat terrain. Watch it carefully if it is shaken it could be due to a broken blade but do not rule out the possibility that the crankshaft of the machine is bent.

Watch this video to learn more about “How to know if the lawn mower crankshaft is bent?”:

How to fix a bent crankshaft?

Damage to the crankshaft is one of the serious problems that must be overcome if the machine is to function normally again. Here are some solutions to fix this problem.

Using the Shaft Straighteners

In this method, we will use a straightener to straighten the bent shaft. We will use mechanical force to act on the shaft in a required position. Can be applied in both hot and cold shafts. This is an extremely effective method to return the shaft to its original state to keep the machine working properly.


This method is more basic and easier than using shaft straighteners but the efficiency is not as high. The only difference between these two methods is that when using a hammer we will apply a direct force on the crankshaft and when using a shaft straightener we will apply a direct force on the shaft.

Replacing a Bent Shaft

In cases where the elbow is too bent to be repaired as it was, this is the last resort. Do not try too hard to repair this part of the machine as it can easily lead to damage to other parts of the machine. When replacing a new shaft make sure the shaft is suitable for your lawnmower motor.

Check How to fix a bent crankshaft on YouTube:

Final thoughts

The bent crankshaft can cause terrible accidents for users. The most common disasters are foot abrasions, scratches, and worse, the loss of toes, half a foot,… To avoid these cases, please pay attention to your mower even if it’s just a small change. The visible signs are unusual noises from the machine.

Although there are some problems during use, the lawnmower usually is an incredibly useful machine. It can help make your landscapes more beautiful. Using these devices is not hard. But it’s vital to recognize any sign as soon as possible.

Please use the mower carefully for the best results. Hope today’s article helps you know: How to tell if lawn mower crankshaft is bent? Have a nice day!

Too Much Oil in Lawn Mower (Symptoms How to Fix)

So you got in a hurry and you overfilled the oil in your mower engine. No big deal right? Wrong.

The manufacturer provides a recommended amount of oil needed for your engine. So why add more than that? If you find yourself doing this, adding too much oil can hurt a lawn mower engine.

Too much engine oil in a lawn mower may cause engine damage due to increased crankcase pressure, overheating, seal damage, blown gaskets, and a hydro-locked engine.

If you catch the problem soon enough, you may be able to fix the excess oil levels to correct the problem. However, when you don’t, you may cause motor damage that will need significant engine work or possibly an engine replacement. Too much oil can hurt the lawn mower engine.

This post may include affiliate links. Purchases made through these links may provide a commission for us, at no extra cost to you. As an Amazon Associate, we earn from qualifying purchases.

Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.

lawn, mower, broken, crankshaft

Negative Effects

Engine Seal Damage

You could damage the oil seals in the engine. This does not sound like significant damage, but if oil blows out of the seals, and you don’t catch it, you can blow up the engine by running out of oil.

When you run out of oil, the excessive heat of the engine will most likely spin a rod or weld itself to the crankshaft.

This heat can also break the rod and send it out to the side of the engine block. The piston can weld itself to the inside of the cylinder causing it to seize.

Engine Overheating

You can overheat the engine and damage your machine when the crankshaft and rod have to push through excess oil. The crankshaft and rod are not able to rotate freely with the extra oil.

  • The crankcase pressure increases and puts the internal working parts of the engine under load.
  • The valve spring and rocker arm will get hot.
  • The oil might not flow properly which can create more heat.
  • Most engines are built out of aluminum which gets very hot.
  • The engine can get so hot it can pop out a valve guide or a valve seat from the engine block. If this happens to your engine you will need to have your engine scraped out.

Engine May Lock Up

When you overfilled lawn mower with oil it may also find its way into the cylinder. This makes the engine turn over very hard. If the oil gets past the rings, due to wear, it could lock up the engine. This is what is known as hydro-locked.

What this means is the combustion chamber, which should only have air and a small amount of fuel in it, now has oil in it.

The piston cannot compress the oil and it becomes hydro-locked. When this happens you can bend the piston rod when you go to start the engine.

Mower Engine May Not Run Well and Smoke

Sometimes when there is too much oil in lawn mower it will continue running, but not very well. Too much engine oil can get up to the spark plug and foul out the spark plug.

The oil inhibits the spark that is needed to fire the fuel properly. Another reason why your lawn mower may not run well is that oil can find its way into the valve train.

This oil gets burned in the cylinder and you will find yourself in a thick Cloud of whitish blue stinky smoke.

In addition to the smoke, your air filter can clog if the engine can’t pull in clean air. The engine may pull crankcase air and oil causing the air filter to plug.

Engine May Blow Gaskets

The engine problems from overfilling with engine oil are mostly the same in twin-cylinder engines. The excess pressure from overfilling may blow gaskets.

Replacing gaskets on a twin-cylinder engine is more expensive than repairs on a push mower engine because you will have to remove the engine from the mower resulting in additional labor costs.

Adding a little additional oil may not seem like a big deal, but it is actually a huge deal that can result in requiring a significant repair or an engine replacement.

So when doing oil changes or adding oil look twice and only add what is required by your engine manufacturer and don’t assume going over the required amount and adding too much oil will be okay.

How to Remove Excess Oil

If you overfilled your engine oil, you need to remove excess oil to correct the oil level as shown on the dipstick.

The first thing you need to do is remove the spark plug wire for safety. There are several different ways to remove excess engine oil from your lawn mower.

    Drain Plug or Valve Port: You need to find the drain plug or valve port on the engine if your lawn mower has one. It may be found on the side of the engine by the dipstick or at the bottom of the oil pan under the lawn mower.

How to Not Overfill Oil

Before you change your lawn mower’s engine oil you need to look at your owner’s manual to find your engine oil capacity.

If you don’t have your owner’s manual, you can always use Google or the search engine of your choice to find the crankcase capacity.

  • Most push mower engines will take about 3/4 of a quart of motor oil.
  • A v-twin engine will usually take approximately 2 quarts.
  • A larger lawn mower commercial engine known as a big block can take 3 quarts of engine oil. These larger engines are known to be about 34 horsepower or larger.

The best way not to add too much oil to the engine is to know your crankcase capacity and only add 1/2 quart of oil at a time. Keep checking the oil until you hit your mark.

If you are adding 1/2 quart at a time, when you get close to being full, just add a little bit at a time and check the oil level on your oil gauge or dipstick.

Will Too Much Oil Prevent a Lawn Mower from Starting?

Too much engine oil can easily damage your machine. The oil pan for the small engines used on lawn mowers is very small.

There is not much room for error with such a small space. If you overfilled the lawn mower with oil the engine can hydro-lock.

What Does Hydro-Lock Mean?

Hydro-locking is when oil gets up into the cylinder and past the piston. The oil then fills up the combustion chamber not allowing the piston to move to the top of the combustion chamber.

The piston is designed to compress air and not oil or water. When you pull on the rope or start the engine with a starter the oil will not compress and it won’t let the piston move. This is referred to as being hydro-locked.

Excess Oil Can Foul the Spark Plug Get Oil in the Carburetor

Too much oil can also foul out the spark plug and not allow the engine to run. Another damage excess oil can do to an engine is oil getting into the carburetor through the valve train causing the engine not to run.

Overfilled oil in a lawn mower can pass through the carburetor and into the air filter which will also cause your engine not to run.

Consult a Small Engine Mechanic

If draining a little oil doesn’t correct the running, starting, or engine problem you’re experiencing from filling the crankcase with too much oil, it may be best to consult a small engine mechanic.

Tearing your engine apart to diagnose the problem may cause additional damage especially when you don’t know what you’re doing.

Find a reputable small engine mechanic to inspect the engine to determine the extent of the damage. Your local full-service lawn mower dealership will be able to perform this service.

You can also search for a small engine repair dealer for the manufacturer of the engine installed in your mower.

Here are a few links to search for repair centers for common small engines used on many common mowers:

Too Much Oil FAQ

Too much oil in a lawn mower engine will cause pressure to increase in the crankcase which may result in overheating, blown gaskets, engine seal damage, smoking, fouled spark plug, clogged air filter, and hydro-locked engine.

Remove the spark plug wire and drain a little engine oil by tipping a push mower, through the drain plug or oil filter on a riding mower, or by using a turkey baster or oil vacuum pump. Once the level is corrected, wipe dripped oil from the mower to prevent smoking.

Still Having Problems with Your Lawn Mower?

Lawn mower ownership doesn’t come without its frustrations. Own a mower long enough, you are bound to run into many lawn mower problems including starting, smoking, leaking, cutting, and overheating.

lawn, mower, broken, crankshaft

For mower troubleshooting, check out my guide Common Lawn Mower Problems: Solved.

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Straighten A Bent Lawn Mower Crankshaft With A Pipe