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Repair: Lawn mower vs. Tree Stump


I was cutting the grass in our back yard with my Craftsman Briggs and Stratton mower when I ran into a little problem.  For the purposes of this story, “little problem” means “tree stump.”  The lawnmower screeched to an abrupt halt as I cursed my own stupidity for hitting an obstacle that I have been successfully avoiding for years.  I was able to re-start it after several attempts but it backfired as it sputtered back to life.  As it was running, it vibrated violently and there were assorted noises coming from the engine that were not there a few minutes before.

Okay.  I did some damage.

Did I just destroy my lawn mower?

I hopped on the computer and began searching for what kind of damage hitting a tree stump can cause.  All the information I found pointed to a bent shaft and that sort of damage was a death sentence for the mower.

Sure, you could try to whack it straight again. There are videos on YouTube of people straightening lawnmower shafts with sledgehammers.  But the more authoritative sites said that most repair shops won’t even attempt this kind of repair because of liability issues.  Once bent, the shaft is weaker, and it might have some microscopic cracks that will worsen over time, resulting in a break that could send the blade flying.

Proponents of the sledgehammer repair all seem to dismiss the danger element.  Their common refrain, “In all my years of fixing lawnmowers like this I’ve never heard of a blade coming off.”

Fair enough.

But the possibility of a lawnmower blade embedding itself in my shin bone after the shaft breaking was enough to persuade me to err on the side of caution.

Buy a new lawn mower. Problem fixed!

I ended up on the Lowe’s website, saw that they had a lawnmower on sale for $100 off.  There were only a couple left in my local store, so from the website, I reserved one for pickup.  All I would have to do is walk into the store, show my receipt and walk out with a shiny new machine.  But that would wait until the next day, Sunday.

In the wee hours of Sunday morning, long before Lowe’s would open their doors, I did some more research and found out that it might not be as bad as I thought.

So, why is my lawnmower running rough and making funny noises after I hit a stump or rock?

There are actually three possibilities.  The worst case scenario is that the impact had bent the shaft. I did a quick visual inspection but did not notice any obvious damage to the shaft. Mind you, I didn’t use a square or measure deviation or anything like that.  It was just a quick look.

More likely, I bent the blade or sheared the flywheel. The cost to fix both of these: about $30.

Bent Blade

The contact with the stump had indeed bent the blade and that could be enough to throw off the balance and cause the motor to run rough.

A new lawnmower blade runs about $20 (2014) and is easy to find in any store that sells lawnmowers.

I needed to change the blade anyway, although I was trying to finish the season with the one I had. After I changed the blade, the mower ran better. There was less vibration, but it the engine was still running rough and making funny noises.

Sheared flywheel key.

This was the repair that scared me because of the amount of dis-assembly.   I found two outstanding videos on YouTube that walk through the process making it less intimidating.

Non-technical explanation of just what the heck happened

There is a notch in the shaft and a notch in the flywheel.  The flywheel key fits in those notches locking the two parts together.  The flywheel key is made of soft metal so that if something causes the shaft to suddenly stop spinning (like, for example, hitting a tree stump), the key will shear to prevent more serious damage to the engine (like, for example, a bent shaft).

How to change a flywheel key

This video by Repair Clinic is a step-by-step tutorial.

Or, if you prefer to see the repair done in real time, then I recommend this video by SmallEngineShop:

O, elusive key, where art thou

I learned that you can’t just walk into Home Depot or Lowes and buy flywheel keys off the shelf like you can with blades.  But surely I could get one from the parts department at Sears, since this was a Craftsman lawn mower.


The parts department would have to order the part and it wouldn’t be in for about two weeks.

Are you kidding me?

The frustrating part of this is that I know that there was likely a tray full of flywheel keys somewhere on the other side of the wall where they do the repairs.  But repair was closed anyway so I wouldn’t even be able to negotiate the purchase of a part out of the repair department’s inventory.   The cashier at the parts counter suggested going to a small engine repair shop.  She handed me a flyer for a shop they recommend– hmmm… maybe Sears no longer does lawnmower repairs in-house after all– but it was Sunday and that shop, and other repair shops like it, were closed. The repair would have to wait.

I walked into the repair shop on Monday, said that I needed a Briggs and Stratton flywheel key, and as soon as I rattled off the first couple of digits of the part number, the guy behind the counter passed one to me.  I was in and out in about a minute and a half.


The videos cover this procedure in detail

I thought taking the lawnmower apart to get to the flywheel assembly was going to be difficult, but it was actually relatively easy.

disassembled lawn mower
The lawn mower disassembled.

The only difficulty I had was removing the nut that secures the flywheel.  The whole assembly spins, so without an impact wrench, you have to find a way to hold the flywheel stationary while undoing the nut.

This nut is easy to remove with an impact wrench.

I don’t have an impact wrench so I propped a piece of wood propped between the discharge chute and the blade to stop the blade from spinning. I was able to remove the nut with a standard socket wrench.

no spin
A piece of wood carefully wedged between the blade and the discharge chute stops everything from spinning.

I did have to make a couple of tool purchases.  The nut is 15/16,  and the largest socket I had was 3/4. That nut has to be tightened to 55 foot pounds so I had to buy a torque wrench as well.   Up to now, I’ve never needed a torque wrench, so I just bought the least expensive one I could find.

The other complication in this repair is the difficulty of removing the flywheel from the shaft.  There is a tool for pulling the flywheel called, wait for it, a flywheel puller.  But instead, I used a prybar to gently nudge the flywheel loose. 

Sure enough, the flywheel key was in two pieces.

flywheel keys
The old flywheel key is on the left, the new one is on the right.

I replaced the flywheel, inserted the new key, reassembled the lawnmower and it fired up as easily as it did when it was new. Success!

Total cost

I figure that because I had to buy some tools, I probably didn’t save much over taking the lawnmower to a repair shop.  But now, I have those tools and the experience to easily make this repair next time. 

Lawnmower blade:  $25

Flywheel Key(s):  $6(I bought 2 so I have an extra one for the next repair)

Gas (used in my vehicle when chasing after the flywheel key):  about $10

Torque wrench: $25

Socket: $8

Total Cost <$75

New Lawnmower >$275.00

Savings: $200

DIY Savings:  The parts would have cost the same. I never got an estimate for the labor cost to replace the flywheel key.  Could I have got it done for $35?  I honestly don’t know.   But with the extra flywheel key on hand, and the new tools in my toolbox, the next time I need to replace the flywheel key, it won’t cost me anything.

The video



  1. Todd Ericsson

    Easily one of the greatest DIY articles in the history of DIY articles. It was actually as if you wrote this specifically for me…and 3 years ago…now THAT’s prescience! Thank you and God bless.

  2. Justin

    I just had a sudden stoppage on a brand new Toro. It runs but it bogs down or dies when I hit wet grass. It is taken a few pulls now to start and just sounds like something is loose. I’m going to give this a shot. I’ve only had the mower a few weeks. Any other pointers or advice?

    • Thumb and Hammer

      I did a Google search for lawn mower noise and there are a few possible causes. Did you actually hit a stump or rock? Considering you are dealing with a new lawn mower, I wold be more inclined to take it to an authorized repair shop. You might get lucky and have part of the repair covered under warranty. It’s a long shot, but there could be some manufacturing defect thing going on here, too.

  3. Danny

    Thanks so much for this…it was exactly what I needed to know to fix my problem!!! My mower blade came to a screeching halt on a big tree root and it was shaking like crazy after that…overheating too. Total cost of my repairs was $30, but that included a brand new blade, 2 replacement housing screws for the ones that literally got shaken off the mower, a pack of 3 flywheel keys, and a new 15/16″ socket. Repairs were relatively easy with the help of your article and the videos. Thanks again!!!

  4. Cathy

    I changed the flywheel key, the air filter, and the spark plug, also cleaned the carburetor, and it acts like it wants to start, but it won’t. Also tested the ignition coil (no spark).

    • Thumb and Hammer

      I wish I could help more, but this blog post is literally everything I know about small engine repair.

  5. Ann

    I had the exact same stump experience this week, but the problem is, I have an electric mower. After I got it started again, the motor tone was lower, gruffer, and the mower vibrates more. The tone dropped from tenor to bass. I have still been able to mow, but it does seem like the batteries are draining faster. I thought about taking the blade off and putting it back on, which is about the only thing I might be able to do on my own. Does an electric motor have a flywheel as a gas mower does, or could it be some other motor issue? I’m worried that if I keep using it I’ll ruin it, but taking it to an electric motor repair shop might be costly, and it will put me out of commission at the wrong time of year. The blade on this OLD mower is a very unique one, and I’m not sure I could find another. I have tried in the past on the parts websites without much luck. Should I stop using the mower, or take my chances? I hate to spend the money right now on a new machine. I like this one because it’s very small and lightweight. Thanks for any advice!

    • Thumb and Hammer

      Unfortunately, the full extent of my knowledge small engine repair is contained in this blog post, so I am going to make a couple of assumptions. I am going to assume that regardless of the power source, lawn mowers are going to work pretty much the same way. The question is whether the flywheel on your lawnmower has a key that is designed to shear in the same way as the the one on the Briggs and Stratton engine. So I looked up an owner’s manual for a popular battery powered lawn mower to see the parts list. It only listed “motor.” Not much help there.

      What has likely happened in your case is that the shaft got bent. If that’s the case, you may not be able to repair it. It might be possible to straighten it, but it could be weakened or damaged and that damage may not be visible to you.

      It is also possible that the blade itself got bent. That would throw off the balance and cause the mower to run rougher.

      Also, if your lawn mower doesn’t have a sacrificial flywheel key, there could be other damage that has been done to the motor.

      You should take a look in your owner’s manual. There’ll be a diagram and a parts list. Otherwise, I would contact the manufacturer directly to see if it even something that can be repaired and ask for authorized repair shops in your area. Not sure how old “OLD” is, but it sounds like you have already done some of this homework.

      I would definitely not use the lawnmower if it isn’t running or sounding right. Could be dangerous.

      My gut instinct is that you will probably be shopping for a new mower. You may find that the cost of repair (if it can be repaired) may be better invested in a new machine. Wish I could be more help. Please let me know what happens. Good luck.


    It is also important to check the blades for these mowers. Sharpen as needed and keep them both tightly fastened. You want to be sure that all lawn mower maintenance that can be done is done so take the time to read your owner’s manual, too. Keep the right products and equipment available to help provide your machine with the best results. If you do not know how to do a repair, be sure that you have a professional do it for you!

  7. Tom

    Well glad i found your site/post thumb-n-hammer.
    Didn’t initially think the key would have been so soft…sure as the sun rises it was crushed.
    Disassembled, reassembled, but with no such luck. Have our daughter cheapie craftsman and will have to either dive much deeper or simply admit defeat and replace

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