This video from Sears PartsDirect shows how to replace a flywheel key on a lawn mower. The flywheel key is a small metal rectangle that keeps the crankshaft and flywheel aligned when you tighten the flywheel nut. To protect more expensive components from damage, the flywheel key is designed to shear if the mower blade hits an object like a rock or tree stump hard enough to make the flywheel slip out of alignment with the crankshaft. If the flywheel key breaks, your lawn mower will stop running or won’t start. You can replace the flywheel key using the steps in this video.
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The flywheel key is a small metal rectangle that keeps the crankshaft and flywheel aligned when you tighten the flywheel nut. To protect more expensive components from damage, the flywheel key is designed to shear if the mower blade hits an object like a rock or a tree stump hard enough to make the flywheel slip out of alignment with the crankshaft. If the flywheel key breaks, your lawn mower will stop running or won’t start. You can replace the flywheel key using the steps in this video. Let the engine cool. Wear work gloves to protect your hands. Now, let’s get started. Disconnect the spark plug wire from the spark plug. Release the starter rope from the lawn mower handle. Remove the screws from the blower housing. Pull the blower housing off the engine. Push the bail control bar down and clamp it to the mower handle to keep the flywheel brake disengaged. Tip the mower on its side with the air filter up. Wedge a block of wood between the mower blade and the housing to keep the crankshaft from turning when you loosen the flywheel nut. Connect a socket wrench to the flywheel nut and tap it with a hammer to loosen the nut. Remove the flywheel nut and washer and pull off the recoil starter cup. To protect the crankshaft thread, rethread the flywheel nut on the crankshaft so it’s flush with the end. Position the flywheel puller tool above the crankshaft on the flywheel and thread the puller bolts into the pre-drilled holes on the flywheel. Hand-tighten the top nuts on the flywheel puller shafts. Loosen the lower nuts on the flywheel puller shafts to allow enough clearance to pull the flywheel up. Alternately, rotate the top flywheel puller nuts clockwise with a wrench to raise the flywheel evenly until the flywheel breaks free from the crankshaft. If the flywheel doesn’t break loose on its own, tap the top of the flywheel with a mallet to break it free. Loosen the top nuts on the flywheel puller shafts. Unthread the flywheel puller shafts from the top of the flywheel and remove the puller tool. Remove the nut and pull the flywheel off the crankshaft. Remove any remnants of the flywheel key from the crankshaft. Line up the notch in the flywheel with the slot in the crankshaft. Push the new flywheel key into the crankshaft slot and tap it down flush with the top of the flywheel. Reinstall the recoil starter cup and flywheel washer on the crankshaft. Thread the flywheel nut onto the crankshaft and hand-tighten it. Reposition the block of wood against the cutting blade so the crankshaft doesn’t turn when you tighten the nut. Use a torque wrench to tighten the flywheel nut to 55 ft.-lbs. Remove the wood block from the cutting blade. Remove the clamp from the bail control arm. Position the blower housing on the top of the engine and line up the mounting holes. Reinstall the blower housing mounting screws. Reconnect the starter rope to the mower handle. Reconnect the spark plug wire to the spark plug.