Can a Shaft be Straightened on B&S 6.75hp Lawnmower

rolmMay 15, 2009

I have a like New Self propelled Troy built mower with a bent shaft it.s a B&S 6.75hp. 2-Qustions Can the shaft be straightened, 2) If not can just the bottom 1/2 crank shaft be replaced. Or is it time for a new engine.

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Not without liability, which is why a shop won't do it. When I bent mine, I removed the spark plug, blade and any adaptor. Rotate the crank and with a piece of chalk, find the high spot. I hit the high spot hard, once, with a light sledge hammer and used chalk again. That's how I fixed mine and it works great. But, like I said, you're on your own as far as liability, if you try this. The shaft can crack and the blade can come off.

    Bookmark   May 15, 2009 at 11:06AM
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Just wondering when you hit the shaft with a hammer will that not damage the crank bearing and the oil seal at the base. Next question I don't understand how you found the high spot with the calk?

    Bookmark   May 15, 2009 at 9:26PM
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Rest your hand on something steady and keep moving the chalk closer until it hits the high spot while the crank is slowly turning. Spark plug out and gas tank empty.

    Bookmark   May 16, 2009 at 7:03AM
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Odds of getting within acceptable specs. with this method are pretty slim. Find a small one man shop that has been in business for a long time and he will likely straighten it for you but finding a shop any more with a straightener is not easy around here. I made my own. While I have heard the broken crankshaft, flying blade story for 60 years, I have only encountered one person who claimed to have actual first hand info on this and that person was on one of these forums, so---. Others were stories about someone down at the corner bar heard so and so had talked to someone who overheard-----.

Walt Conner

    Bookmark   May 16, 2009 at 1:36PM
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If any one knows of where I can get the shaft straightened in the Toronto area Let me know. I do not want to wack the shaft with a hammer. thanks

    Bookmark   May 16, 2009 at 6:24PM
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I have used a crankshaft straightener with good results in a limited way. So far I have done less than a dozen. The first one I did not keep track of where I had applied bending force. I ended up chaseing the bend and broke off the crank. So my question for you more experienced people, what is the correct sequence of steps? How much do you move the shaft past straight to get it to relax back to right? How do you measure how crooked it is? My frame has an indicator. Thanks, Paul

    Bookmark   June 8, 2010 at 1:11PM
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With all due respect to Walt, although I have never personally encountered a crank failure or injury due to straightening, I think this is more than urban legend. Because of liability concerns, I won't do on my own account, or as repair for others.

That said, best way to measure where the crank is out would be to clamp a dial indicator to the deck and rotate crank slowly observing runout. You should have a high and low point 180deg apart and have 1/2 runout at 90deg.

    Bookmark   June 8, 2010 at 1:30PM
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I straightened 2 today and will probably do 2 tomorrow. Not the prefered method by any means but as far as a bent crank the engine is junk. I took off the blade and took off the blade adapter, remove the plug, and as mentioned above i took a pencil and holding it at the end of the shaft as still as i could have someone slowely pull the cord (dont tilt with carb down)and the high point of the shaft will come in contact with the pencil lead and make a small mark. Do this a couple of times. Then slide on an old blade adapter on the shaft and using a good size sledge give it a good wack, Check with pencil after each hit. You will never stand a nickle on it but I would take it home and use it without fear.

    Bookmark   June 8, 2010 at 7:30PM
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