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  #1  
Old 11-20-2018, 02:50 PM
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Question Bend Lawn Mower Crank Shaft /w Hammer?

There are several Youtube videos where push lawn mower bent crankshafts, the part sticking out below the deck and where the blade attaches, that are straightened with several hard strikes with a sledge hammer.

One of my mowers has a bent shaft after the blade hit a low lying tree stump.

There is resultant marked vibration/shaking when running.

The plan is to wrap the exposed crankshaft with tape, insert a 6" length of steel pipe over it, and then strike the end of the pipe furthest from the deck.

The idea is that the pipe will add leverage and also concentrate the force of the strike between the end of the pipe next to the deck and the end of the crankshaft.

BTW, no one in the area does crankshaft straightening.

Any suggestions???
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Old 11-20-2018, 02:56 PM
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Re: Bend Lawn Mower Crank Shaft /w Hammer?

Bring it too the curb or buy a replacement motor. I highly doubt you will ever get it straight enough that it wont destroy the crank bearing anyways, just a matter of time.
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Old 11-20-2018, 05:59 PM
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Question Re: Bend Lawn Mower Crank Shaft /w Hammer?

It'll probably end up at the curb soon (12 years old) so I figure I don't have much to lose with the sledgehammer.

How about heating the shaft with a torch before striking it?

Any blacksmiths out there?

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Bring it to the curb or buy a replacement motor. I highly doubt you will ever get it straight enough that it won't destroy the crank bearing anyways, just a matter of time.
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Old 11-20-2018, 06:44 PM
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Re: Bend Lawn Mower Crank Shaft /w Hammer?

Im not a machinist, but im thinking if you heat it, that might change the tensile of the metal and it might crack or shatter once its running again!
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Old 11-20-2018, 07:50 PM
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Re: Bend Lawn Mower Crank Shaft /w Hammer?

That is certainly true if it gets red hot.

Which I can't do since I just have a propane torch.

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Im not a machinist, but im thinking if you heat it, that might change the tensile of the metal and it might crack or shatter once its running again!
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Old 11-20-2018, 08:16 PM
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Re: Bend Lawn Mower Crank Shaft /w Hammer?

I have done this a few times. Just beat on the blade adapter. My method is use a magnet base for a dial indicator and find the high spot so to speak. Give er a whack and set up the indicator again. Repeat until in spec or till you just give up.

If you have a little luck you end up with a smooth engine and no oil leaks. Crankshaft noses are pretty soft why they bend instead of crack. Not a part I would ever apply heat to with a torch, probably melt the seal then that push to the curb would be a easy decision.

Snapper with Techmuseh (or Briggs can't remember but still have it) engine turned out slick as did a Tractor Supply Honda powered mower.

No loose situation, not worth replacing a crank in an old mower.
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Old 11-20-2018, 10:41 PM
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Re: Bend Lawn Mower Crank Shaft /w Hammer?

I have seen folks bend small engine valves straight enough to be serviceable.

I would not go at it with a hammer though it would take an immense amount of energy to bend a crank however small in dia back to straightishness.

Youre gonna use a pipe can you get one with a thick wall and get its i.d. machined so it barely slides on.

basicall this slide on as far as it goes bend it till it straightens enough to slide the pip on farther. If you can get it seat then hell itll get you mowing again

My great grandfather gustav did it on his bf109 while escaping the russians.
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Old 11-21-2018, 02:51 AM
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Re: Bend Lawn Mower Crank Shaft /w Hammer?

Don't know about mower crankshafts, but I once straightened bent valve heads out of my Alfa Romeo which got bent when I failed to tighten the cam chain idler sprocket.
Removed the head and valves, and while waiting for two new intake valves , thought I would try straightening the bent ones.
Just put the stems in a vice, tapped the heads with a hammer till it was all good. put the valves back in .lapped them in and ran without a problem till I sold the car about over 20 years later.
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Old 11-21-2018, 07:24 AM
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Re: Bend Lawn Mower Crank Shaft /w Hammer?

For the price of a push mower engine replacement I wouldn’t bother, stress crack the shaft and if you have a catastrophic failure, your Ankles could be in bad shape, and you wouldn’t know if it would happen the first time you used it or the 200th
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Old 11-21-2018, 02:38 PM
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Re: Bend Lawn Mower Crank Shaft /w Hammer?

I wouldn't feel to bad about trying to bend it back straight. I've seen the crank on a 16 cylinder 6000 hp compressor drive straightened using that same methodology (bigger hammer and a lot of time spent finding the right spot and supporting the rest of the crank). Just make sure you wear steel toed boots when you cut your lawn after

If it's broke now how can you make it worse?
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Old 11-21-2018, 03:21 PM
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Just make sure you wear steel toed boots when you cut your lawn after

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Old 11-21-2018, 05:09 PM
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Re: Bend Lawn Mower Crank Shaft /w Hammer?

Most of them mowers have an aluminium woodruff key at the flywheel.Aluminium will shear when hitting something hard.That prevents the crankshaft from bending in most cases.

BUT if key is sheared that means the flywheel has turned
on the crankshaft.Flywheel turns then timing is off.Timing off=vibrations that may make an owner believe the crankshaft is bent.

Takes just a few minutes on most mowers to remove the plastic housing then the flywheel nut if just to see what's up.
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Old 11-21-2018, 07:12 PM
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Re: Bend Lawn Mower Crank Shaft /w Hammer?

Many moons ago I worked in a small engine repair shop. I have "repaired" a few with the hammer method. Never had one break off that I knew of. Unless it's a high dollar mower, it is not worth the price of a motor. If it's that old, the rest is about to shit the bed too, wheels cables, controls, engine brake, etc. Dark beer and a hearty smoot with a big club, go for the gusto. Most likely the seal is shot and the lower bearing gone if it was run long after it was bent. Run it till it quits.
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Old 11-21-2018, 08:38 PM
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Thumbs up Re: Bend Lawn Mower Crank Shaft /w Hammer?

Checked and key is intact.

Also the end of the crankshaft where the blade attaches wobbles about 1/8 inch with rotation.



Quote:
Most of them mowers have an aluminium woodruff key at the flywheel.Aluminium will shear when hitting something hard.That prevents the crankshaft from bending in most cases.

BUT if key is sheared that means the flywheel has turned
on the crankshaft.Flywheel turns then timing is off.Timing off=vibrations that may make an owner believe the crankshaft is bent.

Takes just a few minutes on most mowers to remove the plastic housing then the flywheel nut if just to see what's up.
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Old 11-22-2018, 01:19 PM
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Re: Bend Lawn Mower Crank Shaft /w Hammer?

Quit posting and start beating on that thing. The suspense is killing me.



1/8" will beat out nicely. Smooth mowing in your flip flops!
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