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Bent Crank shaft

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  • CarlByrns
    replied
    Originally posted by kbertoson View Post
    Foley-Belsaw did sell a fixture to fix bent crankshafts. They no longer have it in the catalog. Just the pointer and it is on backorder. Your fixture is well done .
    Foley-Belsaw is no longer in the tool business- they're a correspondence school.

    Foley-United sell turf equipment sharpening machinery ( we sell their products), but not crank-straightening tools (probably because of the insurance problem). They have limited support off the older Belsaw stuff.

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  • kbertoson
    replied
    Foley-Belsaw did sell a fixture to fix bent crankshafts. They no longer have it in the catalog. Just the pointer and it is on backorder. Your fixture is well done .

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  • j king
    replied
    I agree. Very nice setup

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  • PStechPaul
    replied
    Very clever! Much more professional than the long pipe and sledgehammer.... But perhaps not as much fun!

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  • Peter S
    replied
    Originally posted by Don Young View Post
    I am not sure who makes (made) it but I have seen a tool which bolts to the crankcase and uses two push screws to straighten the crankshaft and a dial indicator to monitor the results.
    Hey, that sounds like me!

    I read some forum threads about bent lawn mower cranks and saw some photos showing gadgets you can buy for straightening bent crankshafts. I liked the idea of supporting the crankshaft near the main bearing while applying a controllable straightening force.

    The nice thing about having time, lathe, mill and pile of steel is making stuff like this, right?

    The fixture bolts to some existing tapped holes in the crankcase.

    The long hex head bolt is wound in to touch the shaft.

    The socket head capscrew is then wound in to straighten the shaft.

    I put a hole in the fixture for the dial indicator plunger, this means I could check run-out easily.

    I didn't post earlier, because the OP's crankshaft bend sounds more complicated (internal?) than the one I fixed. My shaft still turned freely in the main bearings, the bend was outside the engine.

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    Last edited by Peter S; 12-09-2013, 05:45 AM.

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  • madokie
    replied
    as AK boomer said , you have the perfect staightening jig ,the engine case. i save old cracked cases for this very reason, you dont want to damage a good case by hammering on it, might damage brearing surfaces, so find scoung,or buy at scrap yard busted engine cases. you should be able to get them for scrap prices.or use a hardwood 2x4 with bearing size hole drilled in it, and clamp 2x4 in vise insert bent crank and hammer away.

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  • Don Young
    replied
    I am not sure who makes (made) it but I have seen a tool which bolts to the crankcase and uses two push screws to straighten the crankshaft and a dial indicator to monitor the results. I have straightened crankshafts in the engine with a sledge hammer and also with a press and V blocks out of the engine. The sledge hammer works best if you support the crankshaft near the crankcase on the edge of an anvil or something similar. I am sure it happens but I have never known a straightened crankshaft to break later although I have seen broken crankshafts. I think crankshafts are actually fairly ductile.

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  • A.K. Boomer
    replied
    Lawnmowers are really nice to reverse engineer the bend - just look at what blade took the spanking and at how far out on it's radius - hit same blade/holder on opposite side same distance... it's your best odds for going hack and sometimes hack "gits er done"

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  • Mike279
    replied
    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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  • A.K. Boomer
    replied
    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, 12:43 PM.

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  • Rich Carlstedt
    replied
    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

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  • Willy
    replied
    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.

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  • Duffy
    replied
    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!)

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  • J. R. Williams
    replied
    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

    Leave a comment:


  • 914Wilhelm
    replied
    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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