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Tool Talk Discussion Forum

Bent crankshaft

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jack12345

02-09-2008 21:36:59




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I have a push mower with a B&S engine it has a bent crank is there a way to repair? Homemade tool? Don't want to invest more into buying a tool than the mower is worth.Ideas please. Thanks, Jack




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Kentb of SWMO

02-10-2008 17:59:51




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 Re: Bent crankshaft in reply to jack12345, 02-09-2008 21:36:59  
I have worked on L&G equipment for a number of year. No engine manufacture recommends the way I straiten cranks. There is a fixture made by Sten's for this. I have one BUT you can replace the mower for tha labor involved in R&R the engine and doing the stratening. I use a BIG HAMMER. I tell my costomers that I do not make any warranty on this and that the manufactures DO NOT recommend this way. I may break the crank. All say go ahead and I have NOT broken a crank yet, Knock on Wood. No one has ever brough one back that has broken, Knock on Wood again.

Kent

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JT

02-10-2008 09:51:56




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 Re: Bent crankshaft in reply to jack12345, 02-09-2008 21:36:59  
Jack,
after 30 years of small engine work, this is my opinion. They can be straightened, but when they get bent, it does do some structural damage to the steel, it stretches the steel at the bend when you hit something and stretches again when you straighten it, and will cause metal fatigue, keep in mind, if you hit something again, the crankshaft can break off from metal fatige from the metal being stretched when bending again, and if you hit something with the engine running at 3500 rpm, the blade can become a projectille. If you do decide to take a chance and straighten it, the flywheel key and blade adapter key will be sheared and need to be replaced also.
Jim

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jdemaris

02-10-2008 14:42:28




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 Re: Bent crankshaft in reply to JT, 02-10-2008 09:51:56  
We had several landscaping outfits as customers and they bend a couple of cranks every week.
They were using heavy-duty Bobcat or Sensation push mowers that had pretty rugged blade-hubs. That probably helped with the crank-bending. Granted, the Briggs & Stratton engines are basically "throw away" engines, and not worth doing much work on. But, often straightening a crank is no big deal. I have never seen one break clear off. If you're used to fixing them, you can often do it with one well-placed wack on the crankshaft - and doing so was fully supported by Briggs & Stratton Co. A professional landscaper would rarely get one season out of a new Briggs engine anyway, so longevity was not a big concern. One a crank got good and bent, it was never really right again - it would bend easier the next time - and the bottom main-bearing surface would get scuffed up.

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JT

02-11-2008 09:39:11




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 Re: Bent crankshaft in reply to jdemaris, 02-10-2008 14:42:28  
jdemaris,
I have never actually seem one break either, and when I worked as a distributor rep, Briggs always told us not to striaghten cranks for this reason, I don't know, it makes sense to me about the metal fatique thing, but I have straightened a lot of them with no problems, but with all the lawsuit happy people in this world, I will not do it any more.
Jim



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jdemaris

02-11-2008 10:37:28




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 Re: Bent crankshaft in reply to JT, 02-11-2008 09:39:11  
Yeah, things have really changed. I first worked at a tractor and small engine shop in the late 60s - early 70s. We never gave much thought to getting sued. My boss would buy new, complete unboxed Briggs 3.5 and 4 horse engines by the truckload and we had a back room full of them. Price was around $55 for a 3.5 horse and $70 for a 4 horse as I recall. Those engines would usually last a professinal one season - but -that's cutting grass all day, every day. The big self-propelled mowers we sold were the Buntons and Bobcats - with either 10 horse cast-iron Tecumsehs or 10 - 11 horse aluminum Briggs. Those bigger Briggs still would only last one season. Then - Honda came out with their new 10 horse engines targetted at replacing the Briggs. We were the first dealer in the country to have them. They were outlasting the Briggs three-to-one. Not too long after, Briggs came out with their "IC" series in an attempt to improve their image and quality. It was around that time that I pretty much had moved on to bigger diesel stuff at several Deere dealerships -although I still wound up being in charge of small-engine and chainsaw repairs at every place I worked. Problem was - at the Deere dealerships most of our small-engine line went to homeowners and not landscapers. Personally, I'd rather work on a tractor and deal with a farmer - then fix a lawn mower and deal with an irate and sometimes clueless homeowner.

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JT

02-11-2008 18:30:58




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 Re: Bent crankshaft in reply to jdemaris, 02-11-2008 10:37:28  
Yep, I aggree, people complain more about a $100.00 lawn mower than a $250,000 tractor. I guess it has to do with the $100.00 lawn mower owners think it should run forever. It gets old quick. But as you said, I think it is a lack of education when they buy them from someone who knows even less.
Jim



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jdemaris

02-10-2008 06:12:01




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 Re: Bent crankshaft in reply to jack12345, 02-09-2008 21:36:59  
All you have to do is rap it with a hammer. Once they bend the first time, they bend easier the next. We had to fix hundreds of them. Briggs DOES have a special tool for it. We had one in the shop but rarely used it. Just protect the crank from "hammer-damgage", and rap it good and hard the correct direction and it will go back into place.



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Nellie

02-10-2008 05:01:05




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 Re: Bent crankshaft in reply to jack12345, 02-09-2008 21:36:59  
Sears straightened one for me because it was on warrenty. The engined worked fine for a while and then acted the same as it did when bent.



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moonlite 37

02-10-2008 03:57:29




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 Re: Bent crankshaft in reply to jack12345, 02-09-2008 21:36:59  
I made a straightner from a piece of 8" channel iron to bolt the base of the engine to and had a screw arrangenet to apply pressure. I used a big piston pin to protect the shaft from the pressure of the screw. I got the idea from a factory made one and it works.



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cwtech

02-09-2008 23:18:37




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 Re: Bent crankshaft in reply to jack12345, 02-09-2008 21:36:59  
I worked in a mower shop for 25+ years, and straightened many B&S crankshafts.

Generally, the worse they were bent, the easier they were to straighten.

I don't know which model you have, but the following info generally holds true for a 7/8" diameter shaft. ....If you bent a 1" shaft, I don't think it can be straightened without removing it from the engine.

If the crankshaft is a short one (1" - 1-1/2") don't waste your time trying to straighten it. ....More than likely you will not be successful.

If it is a longer crankshaft (2" or more) these are usually easier to straighten. ...Of course there are exceptions.

At our shop we had a crankshaft straightener. ....This is merely a fixture which the engine bolted to. ....A steel sleeve was slid over the crankshaft, and a jackscrew applied pressure to the bent c/s at its high spot. ....There was a pointer arm which moved on a scale to show the amount of run-out as the crank was turned.

Of course, you don't have a straightener, but if your crankshaft is the long one that extends approx 3", these can usually be straightened with a heavy (5#) hammer while the engine is still on the mower. .....Remove the sparkplug, and ground the plug wire to the engine. ....Remove the blade and the blade holder. .....Rotate the c/s until the high-spot is at the 12:00 position. .....I would use a felt marker to mark the high spot. ....Give the c/s a good solid downward hit near the end of the c/s. A light tap will not move it.

Rotate the c/s to see if you accomplished anything. .....If the high spot is in the same place, you need to give it another hit. .....If the high-spot is opposite the felt mark, then you moved it too much, and will have to move it back the opposite way, but not as much. .....Always keep the high spot at the 12:00 position, and swing the hammer downward.

When you think it looks pretty good, check that the blade isn't bent, and re-install it on the c/s. ....Looking at the ends of the blade in relation to the blade housing while the engine is turned will show how close you are to success. ....If the end of the blade is off by 1/16" or less, you did it well.

Once the shaft is straight, you should replace the lower oil seal on the crankshaft. ....This can be done without removing the engine base.

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Crem

02-10-2008 06:51:50




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 Re: Bent crankshaft in reply to cwtech, 02-09-2008 23:18:37  

Thanks cwteck, for sharing the hints on straightening a crankshaft. A few years ago I hit a post with my lawnmower and bent the shaft. I took the blade off and used an oak block and large hammer and had to hit it pretty hard a number of times to get it straightend. I actually wondered how the bearing could take it. It turned out ok and I am still using it although it is not my main mower. I just use it in rough areas.

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old

02-09-2008 21:53:39




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 Re: Bent crankshaft in reply to jack12345, 02-09-2008 21:36:59  
For the most part the only fix is a new/used crankshaft. BTDT and found its not worth the trouble of trying to make one correct again.



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Jon Hagen

02-10-2008 07:05:26




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 Re: Bent crankshaft in reply to old, 02-09-2008 21:53:39  
Not hard to fix, they bend in the crank pin and twist back in place fairly easy. Find a 6 inch piece of pipe that is a close fit over the crank stub. strike the pipe near the outer end so it puts more twist than side pressure in the shaft and the pipe prevents hammer damage to the crankshaft end. With the sparkplug removed vice grip or bolt a metal gauge strip very close to the crankshaft so you can judge where and how much the shaft is bent. Turn the "high side" to the top, slide the pipe on the shaft and whack it near the outer end with a hammer. Now remove the pipe and reset your gauge so you can see how much you gained and how many more whacks/taps it takes to get it right. Have done many of them, they bend easily and they streighten easily without disassembling the engine.

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Areo

02-10-2008 07:15:25




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 Re: Bent crankshaft in reply to Jon Hagen, 02-10-2008 07:05:26  
Jon, I'm with you, except I use a piece of pipe at least a foot long. As you say, it puts less stress on the bearing and more advantagious leverage on the shaft.

Areo



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JT

02-10-2008 09:57:15




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 Re: Bent crankshaft in reply to Areo, 02-10-2008 07:15:25  
You ever see what happens to a peice of steel that you bend back and forth? And you want to do that with a peice of low strenghth steel with a sharp peice of hardened steel on it while turning 3500 rpm?????



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Jon Hagen

02-10-2008 10:25:44




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 Re: Bent crankshaft in reply to JT, 02-10-2008 09:57:15  
I understand your caution, but I have never seen the actual output shaft part of the crank bend, what seems to bend is the much smaller diameter crank pin inside the crankcase . If the crank breaks at the crankpin, there is little chance the output shaft and blade getting loose from the crankcase.



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