The crankshaft is a central component of your lawnmower’s engine. The piston in the engine is attached to, and drives, the crankshaft. The end of the crankshaft passes through the engine’s crankcase and in the case of most walk-behind lawnmowers, the blade attaches directly to the crankshaft.
Most lawnmower vibration issues concerning the crankshaft are the result of the blade striking a solid object. Tree roots, rocks, buried stakes, or pipes can all cause serious damage to the crankshaft. The tip of the lawnmower blade can travel at upwards of 220 MPH. When the blade strikes a solid object and comes to an abrupt stop, all that energy must go somewhere. Often it is transferred to the crankshaft, which causes it to bend. It doesn’t take much of a bend to cause vibration in the mower – usually just a few thousands of an inch will do it.
The easiest...
The crankshaft is a central component of your lawnmower’s engine. The piston in the engine is attached to, and drives, the crankshaft. The end of the crankshaft passes through the engine’s crankcase and in the case of most walk-behind lawnmowers, the blade attaches directly to the crankshaft.
Most lawnmower vibration issues concerning the crankshaft are the result of the blade striking a solid object. Tree roots, rocks, buried stakes, or pipes can all cause serious damage to the crankshaft. The tip of the lawnmower blade can travel at upwards of 220 MPH. When the blade strikes a solid object and comes to an abrupt stop, all that energy must go somewhere. Often it is transferred to the crankshaft, which causes it to bend. It doesn’t take much of a bend to cause vibration in the mower – usually just a few thousands of an inch will do it.
The easiest way to diagnose a bent crankshaft is to first eliminate any other possible source of vibration. If the mower blade struck something hard enough to bend the crankshaft, the blade will also be badly damaged. So start by replacing the blade with an undamaged one and then test the mower again. If the vibration is still there you will need to do some more complex testing. The next step is to check the crankshaft runout using a dial indicator. First, remove the blade, the blade mounting hub, and the spark plug from the mower. The dial indicator can then be attached to the mower deck and positioned on the crankshaft. Rotate the engine slowly over and read the amount of runout on the dial indicator. The engine manufacturer will have a spec for acceptable crankshaft runout, but as a rule anything over .003” would indicate that damage was done.
Repairing a mower with a bent crankshaft is a major repair and it is not suggested for a novice repair person. However, if you have the skills and the tools to do it yourself, you can save a substantial amount of money. The entire engine will need to be torn down to remove and replace the bent crankshaft. If that seems like more of a repair job than you are comfortable with, you might want to consider taking the mower to a small engine shop for a repair quote. Often having a repair like this performed by a shop will be substantially more expensive than simply replacing the entire engine.
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