A lawnmower crankshaft functions as a bridge between the pistons and flywheel, and is a vital part of starting the engine. The crankshaft requires the up and down movement of the pistons and cranks that the flywheel in a circular movement, which also powers the fan belt and the transmission. Broken crankshafts must be removed before you are able to put in a new crankshaft. A broken crankshaft is among the more expensive lawnmower repairs, but is far cheaper than purchasing a new lawnmower.

Tilt push lawnmowers to drain the gas tank if the tank is attached to the engine assembly; you may want to drain the gas in models with no an engine-mounted fuel tank.

Disconnect the Z-shaped hook end of this engine throttle cable from the carburetor throttle linkage using a set of needle-nosed pliers — grasp the Z-shaped end with the pliers and twist the pliers to remove it from your throttle linkage. Remove the retaining screw, and raise the air filter assembly away from the carburetor.

Unscrew the air filter assembly retaining screw with a flat-head screwdriver and set the screw apart. Pull the air filter assembly off of the lawnmower’s carburetor and set aside for reassembly.

Inspect riding mowers for wire connections which operate to the engine and disconnect the wires. This isn’t necessary for removing push mower crankshafts.

Remove the spark plug from the engine cylinder so that the crankshaft could move without catching on the plug. A spark plug socket wrench may be needed if you can not unscrew the spark plug in hand.

Block the blade using a piece of scrap two-by-four block under the mower deck in order that the blades can not rotate as you work.

Unscrew and remove the push lawnmower blade retaining bolt, lawnmower blade and drier from the crankshaft. When working with a riding lawnmower, you must remove the pulley retaining pulley and bolt from the crankshaft.

Unscrew the retaining bolts which attach the crankcase to the mower chassis using a socket wrench, and raise the engine off of the chassis. Some lawnmowers may have an oil filler tube using a retaining screw that must be removed. Remove the retaining bolts and blower housing from the crankcase assembly.

Eliminate the flywheel retaining nut and use a flywheel puller tool to separate the flywheel from the crankshaft. Line up the bolts at the flywheel puller tool using the flywheel holes. As you tighten the bolts, then the flywheel lifts away from the crankshaft.

Eliminate the crankcase cover — situated on the bottom or close to the engine pulley — from the engine crankcase using a socket wrench.

Place the engine assembly on a flat work surface with the crankshaft facing upward for easy access. Turn the crankshaft and camshaft gears until the timing marks on each equipment line up with each other. Always remove the camshaft from the crankcase without bumping the camshaft from the engine.

Turn the crankshaft to position the piston in the middle of the engine cylinder. Unscrew the retaining bolts and slide the piston connecting rod end cap off of the piston connecting rod.

Turn on the engine crankshaft from the contrary direction of the piston connecting rod to show the broken crankshaft. Slide the crankshaft from the crankcase. Clean any debris from the crankcase with a towel before installing the new crankshaft.