Bent crank + Continued use = Fixed?

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So awhile ago, I posted that my dad bent the crankshaft on his lawn mower by hitting a cement block. He was going to continue to use it until it finally broke as it was knocking like mad! Well a few days ago, he was cutting some tall grass which put some load on the mower and it stopped knocking! I heard it before and it sounded like two metal rods smacking each other and now, it sounds completely normal. What in the world is happening? Has anyone ever heard of this? Oh, a new engine is on the way just in case.
 
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It sounds unsafe. I worked at the Sears repair dept as a 16-17 yr old lol. Even if you got incredibly lucky and it fixed itself, the shaft is weakened and will give eventually. The thought of a blade with a tip speed just under the speed of sound on a weakened shaft doesn't sound good. Running it bent will usually kill the main bearing on that side and show up as an oil leak. Also, the initial impact will shear the flywheel key causing the timing to advance and it will kick back when you try and start it. Are you sure the knocking was not detonation from the timing? It could be the shaft is still bent but the flywheel shifted and the timing is somewhat back to normal.
 
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If it was timing change from a sheared flywheel key, the shaft is not necessarily bent. I don't see how knocking rod could go away. Pull the flywheel off and inspect the key. That could be all that was ever wrong.
 
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Oh, and one more thing... Pay a neighbor kid to mow with it this summer, just in case Just joking...
 

CharlieJ

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I know, I don't see how it could have go away. It is not losing any oil, and he changed the blade on it as it was bent and never mentioned anything about a sheared flywheel key, but I will mention it and check it out. How could a change in timing cause the noise it was making? It sounded like metal on metal contact.
 
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 Originally Posted By: ksJoe
If it was timing change from a sheared flywheel key, the shaft is not necessarily bent. I don't see how knocking rod could go away. Pull the flywheel off and inspect the key. That could be all that was ever wrong.
That's the point. Was it really a rod knock or was it detonation?? I'm not sure from the original post if the shaft was verified to be bent or just the blade. The flywheel key is there for safety and to reduce the chance of bending the crank when you hit stuff. It's aluminum and is meant to be the weak link.
 

CharlieJ

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The blade was replaced right after he hit the cement block and it had a bad vibration with the new blade and it continued to get worse. It got so bad that the knocking was louder than the mower itself.
 
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 Originally Posted By: CharlieJ
The blade was replaced right after he hit the cement block and it had a bad vibration with the new blade and it continued to get worse. It got so bad that the knocking was louder than the mower itself.
Advanced timing will cause the same thing. Did you look at the crank while spinning it over to see if it was bent? You may have gotten lucky.
 

CharlieJ

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No, he probably should have checked that before he ordered a new engine. We will have to check these things before he mows again. Will have running the mower with such advanced timing cause damage?
 
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 Originally Posted By: CharlieJ
No, he probably should have checked that before he ordered a new engine. We will have to check these things before he mows again. Will have running the mower with such advanced timing cause damage?
It pounds the rings, piston, and rod pretty good. Might've taken a little off the life expectancy but since it didn't break anything you're probably good to go. Make sure to let us know what you find. I'm curious now. Good luck!
 

CharlieJ

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I took all the covers off the mower engine today (it is 875 series B&S). Got to the aluminum flywheel, and yep, the aluminum key that holds the flywheel in place with the crank was sheared in two pieces and was jammed in between the threads. I beat on the flywheel a few times and just pulled it off (that was easy). Will get a new key tomorrow and see how it runs. Thanks for the help BuickGN.
 
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CharlieJ

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Ok, it's all back together with the $1 flywheel key and it runs just fine again.
 
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