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  • Bent Crank shaft

    I have a small leaf chipper with an 8 HP single cylinder engine. Something got jammed in the chipper head and stopped it immediately. It now appears the crank shaft is bent. The engine is very strong except for this problem. How difficult would it be to straiten the crank? I have a 30 ton press. Any ideas or suggestions would be appreciated.

  • #2
    I had a similar problem with a vertical shaft lawn mower that hit a boundary marker, it wouldn't run and I was sure it was a bent crankshaft. Turned out that the flywheel had sheared its key and was out of time (magneto magnet in the wrong place at the wrong time). If your crank is bent, it's more likely that it's actually twisted, and the press won't help. Regardless, you're realistically looking at a replacement crank (or engine), straightening it is likely to be an endless iterative process, even if you appear to succeed the probability of sudden, unrecoverable failure in the near future is high.

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    • #3
      I have an 8hp chipper also. I was using it last week and the same thing happened. A piece of a stick got stuck between the part that rotates with the blades and the tin housing next to the motor. Its a bitch to get out. You cant pull the starter to turn it over. But you can remove the cover on the exit shute and get a pry bar in there and rotate it backwards. Yes backwards is how I got mine out.

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      • #4
        It's a bit hard to determine if it can be straightened without actually seeing how it is bent. Was the chipper direct or chain drive? If not, it's odd that the crankshaft bent with a belt drive.

        In any case, the crankshaft may be able to be straightened. You do realize that if it is bent badly enough you have to straighten it BEFORE you try to remove it from the crankcase or you will ruin the bearing bore...unless you are lucky enough to have a crankshaft with ball bearing mains.

        I have made new crankshaft ends to replace thebent part on crankshafts, but it is labor intensive. A new crankshaft may be only $80 to $100 new. It's really only worth making the stub if it's for something that's no longer available.

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        • #5
          I straightened the crankshaft on my brother-in-law's lawn mower using a cheap little 12-ton press. We welded a plate to the table to mount the engine so the crankshaft is under the ram. Find the high spot on the crank and press it down. Check and repeat as necessary.

          The whole operation was surprisingly quick and easy.

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          • #6
            Originally posted by theGallery View Post
            I have a small leaf chipper with an 8 HP single cylinder engine. Something got jammed in the chipper head and stopped it immediately. It now appears the crank shaft is bent. The engine is very strong except for this problem. How difficult would it be to straiten the crank? I have a 30 ton press. Any ideas or suggestions would be appreciated.
            Small engine shops used to straighten cranks all the time. Now their insurance policy won't let them.
            Are you sure the crank is bent? Chipper spec engines have pretty stout cranks- It's more likely that the chopper blade or some of the hammers are bent.

            If it's a Briggs engine, PM me the the model and spec numbers. We sell Briggs parts and I can pull up the part number for you.

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            • #7
              Ahidley, your situation is exactly what happened to mine. It has happened several time over the years and I took the front cover off and got the piece wood out. This time however, I still could not turn the engine over. Yes it is a direct drive. I assumed the chipper head was keyed to the crank. I had to use a puller to get it off. I found the chipper head was not the problem. I can use a pry bar and turn the engine but not easily. The chipper head is NOT keyed to the crank but attached on a taper and I think it is designed to slip on the shaft in this case. I have not dismantled the engine yet but assumed the crank is bent and binding in the bearings causing the rotation problems.

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              • #8
                Originally posted by theGallery View Post
                Ahidley, your situation is exactly what happened to mine. It has happened several time over the years and I took the front cover off and got the piece wood out. This time however, I still could not turn the engine over. Yes it is a direct drive. I assumed the chipper head was keyed to the crank. I had to use a puller to get it off. I found the chipper head was not the problem. I can use a pry bar and turn the engine but not easily. The chipper head is NOT keyed to the crank but attached on a taper and I think it is designed to slip on the shaft in this case. I have not dismantled the engine yet but assumed the crank is bent and binding in the bearings causing the rotation problems.
                If there is binding, dunno, could be bent, but serious binding takes a rather severe bend, unless it has messed up all sorts of things inside. Best not to turn it over until you have a clue what is up.

                The most common problem is that the key in the FLYWHEEL is sheared, or partly sheared. Then the thing won't start r run, and it takes very little movement to cause that. They key can look good. Recently helped neighbor repair lawnmower (posted it last summer, she and daughter had it apart but could not pull the flywheel, I thought they were doing great for folks who had only U-tube to go by). The key looked very close to new, but it wouldn't go at all. New key, and it started in 2 pulls.
                CNC machines only go through the motions

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                • #9
                  Hate to say it but I can go to HF all day and get their horizontal shaft motor for $99 that starts on the first pull and is a good honda clone.

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                  • #10
                    Many years ago I hit a pipe marker with my lawn mower and bent the crankshaft and sheared the key. I replaced the key and with a few well placed blows with a four pound hammer straightened the crankshaft. It ran fine for the next 10 years. Give it a try as most units are ductile iron and will bend very easily. It is no good the way it is now

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                    • #11
                      I have a 5 hp unit and I have abused it SEVERELY over the 20 years I have had it. I seriously doubt the crankshaft is bent. If it is an MTD, then it is a B&S engine and the flywheel/hammer disc/chipper disc is keyed to the crankshaft and retained by a long grade 8 fine-thread bolt. Separate the flywheel housing, remove the bolt, (it will be TIGHT,) and pull the flywheel. Of course, if the bolt is loose, then the flywheel has shifted and THERE is where your trouble lies! (Dont ask me how I know!)
                      Duffy, Gatineau, Quebec

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                      • #12
                        I can use a pry bar and turn the engine but not easily.
                        Not sure what brand of engine you have but most of these engines employ a plain bearing that is part of the crankcase that supports the crank. By forcing the engine to spin when it clearly did not want to you may have ruined the crankcase/bearing.
                        A good rule of thumb is that if the engine will not spin when attempting to use the recoil starter then it's best to find out why first.

                        Hard to say from here exactly how much damage the engine has sustained at this point. The crank is very likely not only bent but very possibly twisted as well. A post-mortem of the crank and it's bearings will soon tell if it will be economically viable to rebuild. Parts prices soon add up to what a new replacement engine can be sourced for.

                        Like 914Wilhelm said, $99 will soon have you in business again.
                        Home, down in the valley behind the Red Angus
                        Bad Decisions Make Good Stories​

                        Location: British Columbia

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                        • #13
                          Many times the bend is outside the crankcase and so the bearing journals are ok.
                          Take two blocks of Aluminum , or brass, or steel and put a piece of paper between them and clamp in your mill vise.
                          Now, bore them to Crank journal size.
                          Then clamp the crank using the milled pieces to hold it in your bench vise.
                          Use the journal closest to the bend for holding.
                          Now take a pipe ( I use a 1 1/2" x 36 " long pipe) and bend the crank back.
                          Use a felt tip marker and mark the low side. so you know which direction to pull the pipe.
                          Protect the threads from damage..
                          Check your work frequently on V blocks to measure your progress.
                          Don't be too aggressive, it will bend
                          Rich
                          Green Bay, WI

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                          • #14
                            Originally posted by J. R. Williams View Post
                            Many years ago I hit a pipe marker with my lawn mower and bent the crankshaft and sheared the key. I replaced the key and with a few well placed blows with a four pound hammer straightened the crankshaft. It ran fine for the next 10 years. Give it a try as most units are ductile iron and will bend very easily. It is no good the way it is now

                            I agree, It's at least my thoughts if your thinking about a new engine - why not give it a try,,, your crank is most likely bent and or twisted right at the connecting rod journal, this in turn has drastically mis-aligned the mains and threw them into a bind,

                            Im in total agreement with Willy stating that there is damage to the mains and by slowly trying to rotate them you will do nothing but create more. BUT, if you "reverse engineer" the bend - and just by using the engines mass/flywheel on the other side --- you can create a situation that instantly lightens the load on the mains and relaxes the bind while at the same time will actually straighten the bend step by step as you keep "upping the ante" of small hand held sledge blows and checking for rotational freedom,,,
                            Is it the right way of doing it? hell no ---------- can it work if your careful? absolutely but don't count on it, it's a hail mary at best but you would be foolish not to try before scrapping as even though the mains seen some abuse mains are resilient compared to the all too feeble connecting rod bearings....

                            In a way you have the perfect jig for holding and straightening - the engine case - just don't try to do it by holding one end of the crank while reefing on the other or you will most likely totally destroy the bearings and might break the case,

                            reverse engineer the forces, use impact in the reverse method- this means somehow finding out where the run-out deviation is and placing your "impact lever" 90 degrees of, it will give you the best odds of bringing things around so not only rotation is free but balance and sealing of crank seals are as close as you can get by this "method"...

                            sometimes turning in the opposite direct will have a freeing up effect if done at the "mass side" (flywheel) - if the other side the direction of run will be more apt to free things up enough to find where the deviation is without damaging the mains to much more,,, it will also allow you the opportunity to "park" the crank to where it will be impacted at the strongest engine case webbing so you run less risk of breaking the case...

                            is it hack? once again hell yes,,,, but sometimes hack gets you back in bizz and outlast everything else, (It's a chipper fer cri sakes - if it's shoots craps nobodies going to "lay it down around a turn")

                            If you go this rout run that pig and heat it up, dump the oil and throw in the finest synthetic money can buy and run that sow till she blows her guts - which may or my not ever happen...

                            keep this in mind - part of that crank wants to go back to where it's once belonged - it takes way less of a nudge to get it there than what took it to deviate from it's norm...

                            also keep this in mind - it will bend easier the next time you get something stuck...

                            feeling lucky???
                            Last edited by A.K. Boomer; 12-07-2013, 11:43 AM.

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                            • #15
                              I got a free 7 hp lawnmower that hit a pipe in the lawn. Bent crank and more than a little. Without removing anything but the blade. Leaving a blade holder on the shaft I used a 8lb sledge hammer to hit the crank. It took more than a few good hits to get it back in line. It may still be off a few thousandths, but it has only required a new blade to run fine the last few years. Very little vibration and since I did not get to see it run before, I doubt it was any smoother running. I would give hitting it a try first and see what you get. Free is always a good first step in the repair process. And yes it did shear the flywheel key. Mike

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