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.”
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.
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.
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.
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.
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.
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!
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
Total Cost <$75
New Lawnmower >$275.00
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.