Think I bent the crankshaft on the mower

[ragtoplvr]

Originally Posted By: Cujet
I use a 5 pound sledge and a dial indicator to straighten the mower crankshaft. Works perfectly.
+1000 Years ago In college I made good side money fixing mowers with broken ropes, bent cranks,sheared flywheel keys, and cracked blade adapters. There is a "Feel" to this, not everyone can do it. If the spark plug fires exactly as the blade hits, you can bend a rod. I saw this happen once. That mower donated parts.

Rod

 

[Kestas]

Originally Posted By: Lubener
... Don't attempt to have a crank straightened. The metal will be weakened and that's something I wouldn't want when a blade is spinning at 3000 RPM's...

As long as you don't develop a crack during straightening, you'll be fine. The metal will not weaken.

 

[ragtoplvr]

I mowed yards. After I straightened one I always used it a couple of times. Never had one fail. Then I would finish a service clean deck, sharpen blade, oil wheels etc and sell. Most mowers I got for free, or for $5.00 and sold for $40 to $60 depending. This was in the 70's so good money. BTW only saw one Lawnboy with a bent crank, and it also broke the mounting ears off the block.

Used the deck on another one.

Rod

 

[Merkava_4]

Originally Posted By: Lubener
Anyone who has never seen a bent crank has not had much experience working on small engines.


I have never seen one, but then again, horizontal shaft engines don't have that problem!

 

[dlundblad]

You should be able to find a 2 stroke Lawn Boy on craigslist for around $50. Chances are, it will be built better too.

Unless doing it as a hobby (or sentimental reasons), straightening the crank or replacing a $100 lawn mower engine isn't too cost effective when used can be had for roughly half that.

 

[yeehaw1960]

Because it starts right up and runs, your flywheel key is not sheared. It would take a monsterous blow to shear the blade key.

Clamp off gas or turn off shut off valve. Run engine out of gas if possible. Remove the air filter. On a Honda, I remove the oil too. Remove the plug and tie off the stop cable (dummy) bar so the engine will turn over easily.
Turn mower on its side with the muffler up.
Remove blade and blade mount.
Pull rope to see which way the shaft is bent. I tie a string on one corner of the frame and run it near the shaft to the opposite corner (poor man's dial indicator).
Strike (with authority ONCE) on the high side and pull the rope again.
Repeat until shaft is as straight as you can get it. You will absolutely have to settle for "good enough". Sometimes whacking a stump or the process of straightening the shaft will cost you the bottom crankshaft seal--easy fix.
Put Blade mount back on (may have to grind/file out the keyway a little after beating on the shaft). Put on new blade or sharpen the old one (if it isn't bent).
If it won't start or pulls back on the rope when trying to start it, the flywheel key is probably sheared. Let us know how it goes. It will never be perfect again, but it should go from exorcist to mild vibration.
NEVER EVER EVER heat the shaft to bend it back into shape...something to do with temper and danger of shaft shattering or worse. I guess unless you're a blacksmith or machinist who can assess the actual risk.
And they all lived happily ever after!

 

[bubbatime]

Originally Posted By: Merkava_4
Why not replace the crankshaft?

And while you're at it, replace the crankshaft oil seals and install new piston rings.


The GC engine is a throw away engine. You don't replace crankshafts on them. Well you could, but you would be spending about as much as a new engine, so just buy the new engine.

To the OP, some mower shops have crankshaft straighteners (hydraulic press and dial indicator). I'm not sure what they charge for this service.

 

[bubbatime]

Originally Posted By: yeehaw1960
Because it starts right up and runs, your flywheel key is not sheared. It would take a monstrous blow to shear the blade key.


Actually hitting a stump with the blade is enough to shear a flywheel key. Happens all the time. Many engines will start up and run fine with a sheared fly wheel key, but will sometimes run erratically.

 

[bubbatime]

Originally Posted By: Panzerman
Iam talking about the key way on the flywheel. It will run and it will shake and vibrate. I seriously doubt you bent the crank. I would have to see a crank bent to believe it. I just don't think the horsepower is there. It will shear the key way off or just stop.


Cranks bend all the time when the blade hits a stump. This isn't new or earth shattering news. It happens. All the time. Horsepower doesn't matter. The engine is running at 3100 rpm, which is easily enough force to bend a crankshaft.

Most likely the op has a bent crankshaft.

 

[yeehaw1960]

I guess I'll just have to wait until I get a little more experience with small engines before I comment. If you were in Omaha, you could come over and pick up one of the 5 or 6 used 160s I have out in my garage. Just follow the instructions I typed out above and you'll probably become a happier camper.