Bent crankshaft on 4.5hp B&S lawn mower - HobbyTalk
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post #1 of 30 (permalink) Old 07-14-2007, 04:03 AM Thread Starter
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Bent crankshaft on 4.5hp B&S lawn mower

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.

Last edited by sluggermike; 07-14-2007 at 04:06 AM.
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post #2 of 30 (permalink) Old 07-14-2007, 12:57 PM
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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.

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post #3 of 30 (permalink) Old 07-14-2007, 02:21 PM Thread Starter
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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?
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post #4 of 30 (permalink) Old 07-14-2007, 03:25 PM
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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.
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post #5 of 30 (permalink) Old 07-14-2007, 04:41 PM
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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.
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post #6 of 30 (permalink) Old 07-14-2007, 05:35 PM Thread Starter
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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.
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post #7 of 30 (permalink) Old 07-14-2007, 06:27 PM
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deleted, apparently I'm just a troll.

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post #8 of 30 (permalink) Old 07-14-2007, 06:56 PM Thread Starter
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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?
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post #9 of 30 (permalink) Old 07-14-2007, 09:20 PM
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Bent Crankshaft

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.
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post #10 of 30 (permalink) Old 07-14-2007, 11:18 PM Thread Starter
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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.
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post #11 of 30 (permalink) Old 07-14-2007, 11:21 PM
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Part Number and suggested list price from Sears.com

691472 CRANKSHAFT $45.41
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post #12 of 30 (permalink) Old 07-14-2007, 11:40 PM Thread Starter
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Thanks for the informatio on the crank. Is a 1/8" deviation OK?
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post #13 of 30 (permalink) Old 07-15-2007, 08:22 AM
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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.
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post #14 of 30 (permalink) Old 07-18-2007, 06:34 PM Thread Starter
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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?
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post #15 of 30 (permalink) Old 07-18-2007, 06:49 PM
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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.
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