View Full Version : Bent crankshaft on 4.5hp B&S lawn mower


sluggermike
07-14-2007, 04:03 AM
I inherited a Craftsman lawn mower. I got it to run, but not well. I was told that it has a bent crankshaft which I confirmed when I looked under the deck. When I pulled the started rope, the recoil pulled it out of my hand like it was backfiring. In an earlier post someone wrote that the crankshaft could be straightened while still in the engine? If it can't be strightened while in the engine, can it be straightened if it is taken out, and how difficult is that to do? I would suspect that a machine shop would be the place to do this type of work? Thank you for your help.

30yearTech
07-14-2007, 12:57 PM
No, crankshafts should really not be straightened. You run the risk of the crankshaft breaking while running and then you can have the mower blade come flying out from underneath the mower deck.

If you do the work yourself, you can replace the crankshaft with a brand new one for about the same price or less then the time it would take to straighten it. If you are paying someone to replace it then that might make a difference. If the mower is a low end unit then it's probably not worth the time or effort to repair it, unless it's just something you want to do.

:thumbsup:

sluggermike
07-14-2007, 02:21 PM
Thanks for your advice. Do you have any idea how much a crank would cost? It's not a high end lawn mower, but it looks fairly new and this is something that I would like to do so I can learn. I was wondering if the impact that bent the crank caused the timing to be off which would explain why the rope was ripped out of my hand when I tried to start it. I mentioned in the post that it did run, but poorly. I won't try to start it again until I do something about the crank. How do you check the timing?

justin3
07-14-2007, 03:25 PM
The timing issure is a sheered flywheel key, when the blade had its impact the sudden stop of the flywheel will sheer the key. Easy 99 cent part to replace.

30yearTech
07-14-2007, 04:41 PM
Crankshafts run around $45.00 - $65.00 for many of these engines, could be a little more just really depends on which crankshaft your engine uses. Post the model an spec numbers and I can find out which crankshaft your engine uses.

Justin3 is correct, the flywheel key is probably sheared on your engine, this causes the spark to be advanced and thats what makes the rope jerk out of your hand when you try to start the engine. Most likely is the reason for the engine to not run good.

sluggermike
07-14-2007, 05:35 PM
Thanks for the information. I'm beginning to wonder if the problem is the timing and not a bent crankshaft. I will check out the flywheel key and look closer at the crank. The end of the crank under the lawn mower deck appeared to be a little out of line, but it was not leaking oil from the seal which I would expect. I will check it with a stright edge.

newz7151
07-14-2007, 06:27 PM
deleted, apparently I'm just a troll.

sluggermike
07-14-2007, 06:56 PM
I checked the flywheel key and it is obviously mangled which explains the timing problem and why the started rope was pulled out of my hand. I will try your suggestion about having someone pull the started cord while I watch to see how badly it is bent. I've never checked other mowers, and so I was wondering if any degree of bend or deflection is acceptable?

30yearTech
07-14-2007, 09:20 PM
Any bend at all will affect the quality of cut. But since this is not a high end mower that may not make much difference. The crankshaft can be bent quite a bit without any oil leaking from the oil seal, most shafts are stepped down and bend below the seal as there is considerable support just above the seal.

Running a mower with a bent shaft can result in excess vibration which can cause hardware to loose and sheet metal to fatigue over a period of time. If the shaft is bent and you want to run it that way there should be no real problem, just keep an eye on the nuts and bolts and keep them tight. Retire the unit if any fractures appear in the mower deck.

sluggermike
07-14-2007, 11:18 PM
Thank you'll for helping. I checked the bend with a friend of mine. There is a 1/8" deviation. It sounds like it should be OK as long as that isn't too much of a bend. Just in case, I decide in the future to get another crank, it is a Craftsman 4.5 hp, model 10L902. type 0488.

30yearTech
07-14-2007, 11:21 PM
Part Number and suggested list price from Sears.com

691472 CRANKSHAFT $45.41

sluggermike
07-14-2007, 11:40 PM
Thanks for the informatio on the crank. Is a 1/8" deviation OK?

30yearTech
07-15-2007, 08:22 AM
You would just have to run it and cut with it to see how much vibration there may be and how the quality of cut is.

I have seen mowers being used that have quite a bend in the shaft. It's really up to you to make the determination of whether to use like this or not. If it's a really nice mower then I would probably fix it, if it's not then I would use it like it is until it's used up.

sluggermike
07-18-2007, 06:34 PM
This engine seems to have a mind of it's own. I pulled the flywheel becauses I thought it had a bent flywheel key. It didn't. I replaced the flywheel and the same key and then tried to start it again. Not to my surprise it kicked back again when I pullled the starter cord. I'm still wondering if the timing is off, but I don't know how it could be since the slot on the flywheel and the crank lined up for the key. Any ideas?

30yearTech
07-18-2007, 06:49 PM
Check the blade and make sure it's good and tight, the mower blade acts as a counter balance and if the blade is just a little loose, the engine may kick back when you try to start.

engine man
07-18-2007, 09:34 PM
i have a crankshaft straightener and have used it for 4 years now and never had a problem with broken cranks. it is very easy to use and saves you money.

30yearTech
07-18-2007, 09:51 PM
i have a crankshaft straightener and have used it for 4 years now and never had a problem with broken cranks. it is very easy to use and saves you money.

http://www.hobbytalk.com/bbs1/showthread.php?t=178229

This subject was discussed awhile back in the thread listed above. It's certainly up to you if you want to take your chances straightening out a shaft, I just would not recommend it. The chances are probably slim that you will have a crankshaft break, but it only takes one that results in an injury to come back and haunt you for the rest of your days.

Most commercial insurance policy's prohibit this type of repair, as well as dealer agreements with the engine's manufacturers.

scrench
07-18-2007, 11:25 PM
Originally Posted by engine man
i have a crankshaft straightener and have used it for 4 years now and never had a problem with broken cranks. it is very easy to use and saves you money.


i have one too its called a 2 # hammer , i have never had any problems my self i only use it on my own stuff , would never ever ever ever do it on someones mower other than my own , i thought crank straighteners were out lawed ? maby maby not ?

sluggermike
07-19-2007, 05:01 AM
Thanks, in fact, I don't have a blade on it. I will put one on and let you know what happens.

justin3
07-19-2007, 07:43 AM
Yeah I learned the hard way that you can't start a lawnmower engine without a blade, almost broke my wrist. But that was at age 10, 4 years ago.

engine man
07-19-2007, 06:44 PM
i didnt know you couldnt start it without a blade? its very hard to pull over but i start them sometimes without a blade just to see if its in running condition.

30yearTech
07-19-2007, 09:28 PM
You can start them without a blade, but you have to be ready for it. You have to pull it really fast for it to start, the ignition timing is set slightly advanced on these engines, so the plug fires before the piston gets to top dead center.
The inertia of the blade spinning keeps the engine rotating in the same direction. Without the counter balance of the blade or some kind of flyweight then when the spark ignites the fuel, the engine tries to fire up backwards.

sluggermike
07-20-2007, 12:53 AM
I put a blade on this afternoon and it worked. There is some vibration, but it doesn't seem to be excessive. I suspect all lawn mowers vibrate to some degree. I've learned a lot from this exercise, and I appreciate all the help. Thanks.

sluggermike
07-20-2007, 07:07 PM
I'm curious what does a crankcase strightener look like and where can you get one. The only thing I could find online was a hair straightener and I don't need that.
Thanks.

30yearTech
07-20-2007, 07:36 PM
I'm curious what does a crankcase strightener look like and where can you get one. The only thing I could find online was a hair straightener and I don't need that.
Thanks.


http://www.smallgasengine.com/Crankshaft-Straightener/pages/Diaphragm.asp

sluggermike
07-21-2007, 07:13 PM
Thanks, 30yeartech. I really appreciate your help and everyone elses. I've learned a lot.

MacEarl
04-20-2011, 05:22 PM
Originally Posted by engine man
i have a crankshaft straightener and have used it for 4 years now and never had a problem with broken cranks. it is very easy to use and saves you money.


i have one too its called a 2 # hammer , i have never had any problems my self i only use it on my own stuff , would never ever ever ever do it on someones mower other than my own , i thought crank straighteners were out lawed ? maby maby not ?

There is nothing wrong with straightening a slightly bent crankshaft. Great big automobile, truck, locomotive and other crankshafts are routinely straightened. But not much.

Only YOU know how many times and by how much your crankshaft has been bent and straightened. This is a major reason why shops no longer straighten bent cranks. They have no idea how much your crank has been bent before and how many times it has been straightened.

A new crankshaft for my Lawn-Boy M is $275. No way I will pay anything like that for my 20 year old mower.
Apparently it bent at the throw (big end crankpin) as the flywheel wobbles about as much as the blade. The blade wobbles about 1/8" at the tips.

I had a crankshaft extension (the part that reaches down to the blade) break off up in the seal. The mower had never hit anything. The engine suddenly raced, then stopped. I could not figure out what went wrong, as when I started it, it would fire, race a few times and then stop. I turned the mower up and found that the entire blade and shaft were missing. I found it out in the grass where it had first stopped. It did not throw the blade; it just dropped to the ground.
After all, the blade is balanced. It's not like one half of the blade broke off and went flying.

MacEarl
04-20-2011, 05:26 PM
Originally Posted by engine man
i have a crankshaft straightener and have used it for 4 years now and never had a problem with broken cranks. it is very easy to use and saves you money.

i have one too its called a 2 # hammer , i have never had any problems my self i only use it on my own stuff , would never ever ever ever do it on someones mower other than my own , i thought crank straighteners were out lawed ? maby maby not ?

Hammering on the crankshaft extension is mighty hard on the ball bearings of a 2 stroke engine crank.

It's shop liability insurance that has forbidden straightening. They exclude damages from broken crankshafts from coverage if they can prove the crankshaft had been straightened.

30yearTech
04-20-2011, 08:20 PM
There is nothing wrong with straightening a slightly bent crankshaft.

Yes there is!! You don't have a blade spinning at 17,000 feet per minute on the end of a automotive crankshaft. If a crankshaft breaks, there is a possibility that the blade can come out and injure the operator or a bystander. If your alright with the risk, then do it and take your chances. Encouraging this type of repair is irresponsible.

geogrubb
04-20-2011, 10:02 PM
For my $0.02 I refuse to work on a mower with a bent crank, why take a chance on someone or something getting hurt. Have a good one. Geo