Sometimes you see large, concealed objects like stumps and rocks at the last moment, which makes it hard to avoid hitting them while you mow your lawn. This not only damages the cutting blade, but can also split the engine crankshaft and other internal components. A broken engine crankshaft is an expensive fix, and it takes basic hand tools and a couple of specialty tools to replace. You have to consult with the lawn mower service repair manual for a reference manual for torque specifications.
Drain extra fuel in the fuel tank of an engine that has the fuel tank mounted on the engine assembly. Unhook the engine throttle cable’s Z-shaped finish in the carburetor throttle linkage using needle-nose pliers. Remove the retaining screw, and raise the air filter assembly away in the carburetor.
Unhook any wire connectors that may be running to the engine on riding mower models. Unscrew the engine spark plug in the engine cylinder with a spark plug socket wrench to permit the engine crankshaft to move freely — that applies to riding mowers and push mowers. Insert a wood block between the mower deck and cutting blade border to block the blade from turning.
Loosen the blade retaining bolt with a socket wrench, and pull the blade and drier from the crankshaft on push mowers. On riding mowers, unscrew the pulley retaining bolt, and eliminate the engine pulley in the crankshaft. Drain the oil in the engine.
Unscrew the crankcase-to-chassis retaining bolts using a socket wrench. Lift the engine in the chassis and set it on a good work surface. Unscrew the oil filter tube retaining screw with a socket wrench, if required. Unscrew the blower housing retaining bolts from the engine crankcase assembly, and raise the compressor housing from the engine crankcase assembly.
Unscrew the flywheel retaining nut in the crankshaft using a socket wrench. Pull the flywheel in the engine crankshaft using a flywheel puller tool.
Unscrew the crankcase cover retaining bolts in the engine crankcase using a socket wrench, and remove the cover in the crankcase. The crankcase cap is generally on the bottom or blade and pulley side of the engine. Put the engine assembly on its side, with all the vulnerable camshaft and crankshaft facing upwards.
Expand the timing marks on the crankshaft gears using the timing marks on the camshaft gears. Pull the camshaft gently in the engine crankcase to avoid bumping or hitting the camshaft lobes on the engine.
Rotate the engine crankshaft until the piston reaches middle from the engine cylinder. Unscrew the piston connecting rod end cap retaining bolts using a socket wrench; pull on the piston connecting rod end cap in the connecting rod.
Rotate the crankshaft away in the piston connecting rod; pull on the broken crankshaft in the crankcase. Inspect the interior of the crankcase for damage or debris left behind from the broken crankshaft, and wash it off with a shop towel.
Apply a coat of engine oil to the crankshaft oil seal with your finger to lubricate the seal. Slide the new crankshaft to the engine oil seal and crankcase carefully to avoid damaging the crankshaft lobes and journals.
Rotate the crankshaft to bench the crankshaft lobe to the piston connecting rod finish. Set the end cap on the piston connecting rod; torque the finish cover retaining bolts with a torque wrench and socket to the specifications to your lawn mower model.
Slide the camshaft to the crankcase, aligning the timing marks on the camshaft gear and crankshaft gears. Slide a new crankcase cap gasket above the crankcase, aligning the crankcase gasket holes using the retaining bolt holes. Slide the crankcase cover above the engine crankshaft, flush with the crankcase, and torque the crankcase cover bolts.
Assemble the lawnmower engine in reverse order of disassembly and put it at the lawnmower chassis. Torque the crankcase retaining bolts with a torque wrench and socket. Hook the throttle cable’s Z-shaped end on the carburetor throttle linkage using needle-nose pliers. Set the air filter assembly on the engine carburetor, and tighten the retaining screw with a screwdriver.
Reconnect any electrical wires that you disconnected from the engine. Fill out the engine with fresh engine oil. Reinstall the spark plugin; tighten it using a spark plug socket wrench, and rebound the spark plug wire.
Initiate the lawnmower engine and allow it to idle for approximately 30 minutes to break in the new crankshaft. Drain the engine oil to eliminate small metallic particles that can damage the engine, and refill with new oil.