How to Straighten a Bent Lawn Mower Shaft
By Meredith Jameson
The shaft on a lawn mower connects the blade to the motor and can easily be bent if the lawn mower hits a large or hard object, such as a tree stump, rock or any other large debris. Once the lawn mower shaft is bent, the lawn motor may vibrate excessively and other problems such as problems with the motor or transmission might occur. Straightening a bent shaft is possible, but owners should beware that doing so might invalidate their warranty unless the mower is serviced by an authorized service center.
Accessing the Shaft
Allow the lawn mower to cool completely if recently used.
Place an old towel on the ground to protect the ground and the lawn mower and a drain pan under the mower. Unscrew the oil drain plug and allow the oil to drain completely.
Close the fuel hose with a clamp next to the fuel tank and then disconnect the fuel hose. Open the gas cap and drain the fuel out of the mower.
Disconnect and remove the air filter assembly and the fuel tank. Unhook the engine cables and throttle cable with a Philips-head screwdriver and then pull out the oil tube dipstick. Disconnect the metal housing on top of the engine and the spark plug wire. Remove the spark plug.
Hit the starter clutch carefully with a rubber hammer counterclockwise until loosened and then remove the clutch. Slide a small piece of wood between the flywheel and the housing and then loosen the bolt on the flywheel and remove by placing the flat blade of a screwdriver between the flywheel and the housing. Tap with the flywheel with the rubber hammer and gently pry loose with the screwdriver blade.
Pull out the shear pin from the crankshaft. Grab a flat side of the lawn mower blade with one hand and turn the bolt on the blade with a large wrench counterclockwise. Pull off the blade and the bolt and set aside.
Return the lawn mower to the upright position and disconnect the bolts that hold the deck and the engine in place. Pull out the motor and set aside.
Find the valve spring cover and disconnect the screws holding the cover in place. Place a valve spring compressor tool on the valve spring between the metal washer and the valve spring, then push down on the spring. Move the metal washer up and off while the spring is compressed and then carefully let go of the spring. Repeat on the other side with the second valve spring.
Slide the engine case off by the blade and then pull out the camshaft gear. Push down on the tabs where the rod end cap bolts are connected and unscrew the bolts to remove the rod end cap.
Push down on the connecting rod and move the rod up until the cylinder piston moves to the top, then turn the crankshaft slowly. Move the crankshaft off the engine when the connecting rod pin is pointing away from cylinder.
Straightening the Shaft
Place the bent shaft on a sturdy, hard surface that will not be damaged during shaft repair.
Set a length of heavy pipe over the bent section of the shaft.
Hammer the pipe over the shaft two or three times and then remove the pipe.
Check the straightness of the shaft using a level and repeat the hammering process as necessary until the shaft has largely resumed a straight form.
Reverse the disassembly process to put the mower back together.
- Do not attempt this task if you are not confident in your ability to do so safely and effectively. Contact the lawn mower manufacturer or another qualified service professional for assistance instead.
Meredith Jameson writes early childhood parenting and family health articles for various online publications. She holds a Bachelor of Arts in history from San Francisco State University.