Replacing a Flywheel Key on a Riding Lawn Mower

This video explains how to replace the flywheel key on a riding lawn mower or lawn tractor. The flywheel key is a small rectangular piece of metal that keeps the crankshaft and flywheel aligned when you tighten the flywheel nut. To protect expensive engine components from damage, the flywheel key shears off if the mower blade hits an object hard enough to make the flywheel slip out of alignment with the crankshaft. Replace a broken or sheared flywheel key with a part that fits your model.

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The video includes these parts and tools. Look up your model to make sure you get the right parts for your mower.
Flywheel key
Flywheel puller

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The first step to replacing the flywheel key is to turn the ignition switch off and remove the key. Lift the seat. Remove the bolt connecting the negative cable to the battery and tuck the battery cable away so it doesn’t touch the battery post. Lift the mower hood. We’ve removed the hood to give you a better view. Remove the air duct mounting screws and pull off the air duct. Rotate each air filter cover retaining knob counterclockwise and pull off the air filter cover. Remove the air filter. Remove the air filter housing screw. Remove the front blower housing mounting bolts. Remove the rear blower housing mounting bolts. Lift off the blower housing. Remove the flywheel screen cap. Disconnect the spark plug wire and remove the spark plug. Insert nylon rope into the spark plug hole to jam the piston and keep the crankshaft from turning while you remove the flywheel bolt. Leave some rope hanging out of the cylinder so it’s easy to remove later. Remove the flywheel bolt, washer and flywheel screen. Pull off the flywheel fan. Thread the flywheel bolt onto the crankshaft. Position the flywheel puller tool above the flywheel bolt and thread the puller bolts into the pre-drilled holes on the flywheel. Turn one flywheel puller bolts clockwise to tighten it slightly, and then slightly tighten the other one. Alternate between the two to screw in the bolts, raising the flywheel evenly until the flywheel breaks free from the crankshaft. After freeing the flywheel, remove the flywheel puller and flywheel bolt. Pull the flywheel off the crankshaft. Remove the remnants of the flywheel key from the crankshaft and flywheel. Push the flywheel onto the crankshaft and line up the notch in the flywheel with the slot in the crankshaft. Insert the new flywheel key into the crankshaft slot and push it down flush with the top of the crankshaft. Reinstall the flywheel fan and screen. Position the washer over the crankshaft hole. Thread the flywheel bolt into the crankshaft and hand tighten it. Use a torque wrench to tighten the flywheel nut to 100 ft-lbs. Pull the rope out of the engine cylinder. Reinstall the spark plug and reconnect the spark plug wire. Reinstall the flywheel screen cap. Position the blower housing on the engine and align the air diverter in the housing. Reinstall the blower housing mounting bolts. Reinstall the air filter housing screw. Reinstall the air filter. Position the air filter cover on the engine and turn each retaining knob clockwise on the cover. Reinstall the air duct and secure it with the mounting screws. Reconnect the negative cable to the negative battery terminal. Lower the seat.



5 thoughts on “Replacing a Flywheel Key on a Riding Lawn Mower

  1. The original key lasted 15 years.
    I can't torque it down anymore with a pipe wrench holding the pulley shatf under and an 18 inch Crescent wrench on top.

  2. I have an 18 h.p. B&S on a Craftmans LT 1000 model 31H777. I normally start my mower in the garage. My flywheel key breaks as soon as I start the mower. I have replaced 3 keys in last 3 days.
    I replaced the crankshaft, piston rod, and set the valves to specs.
    Why would the key break like that with no load on?
    Is it caused by cheap keys?

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