How to Replace the Flywheel Key on a Briggs & Stratton

Written by g.k. bayne
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The flywheel key is an important part of the Briggs & Stratton engine. It is part of the twofold timing system that allows the engine to operate properly. Generally, the half-moon-shaped flywheel key can be sheared when the crankshaft is stopped abruptly. In the case of a lawnmower, this can occur when a stump or rock is hit by the grass cutting blade. Replacing the flywheel key on a Briggs & Stratton engine may take a couple of hours, but it is time well-spent and will keep some needed cash in your pocket.

Skill level:

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Things you need

  • 7/16-inch and ½-inch end wrench
  • Flywheel wrench or small diameter long bar
  • Channel lock pliers
  • Wire cutters
  • Mill file
  • Emery cloth
  • Grease
  • Hammer

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  1. 1

    Remove the pull starter cover that conceals the flywheel on the engine. There are four bolts; depending on the engine size, the 7/16-inch or ½-inch end wrench will fit the heads of the bolts.

  2. 2

    Pull the starter cover off the engine and place the four bolts inside the cover so you will not lose them.

  3. 3

    Place either the flywheel wrench or the small diameter long bar between the cooling fins of the flywheel. You will note that six of the fins are thicker than the other vertical fins. Theses heavier air fins should be engaged with the flywheel wrench or small diameter long bar for the removal of the spring dog clutch.

  4. 4

    Using the channel lock pliers, remove the spring dog clutch in a counterclockwise direction while holding the flywheel stationary with the wrench or bar. The clutch will "break" loose, and you can finish unscrewing the clutch assembly from the crankshaft with your fingers. Be careful how much force you apply to the fins if you are using a small-diameter long bar. The fins can be brittle, and may break from the excessive force placed on them from the bar.

  5. 5

    Pull the flywheel from the shaft and use the wire cutters to grasp hold of either side of the flywheel key. Brace the rear of the cutters against the crankshaft and "pry" the key loose from the machined slot in the shaft. The key should pull freely from the slot, with little effort.

  6. 6

    Use a small mill file to gently file (dress down) any raised ridges that may have formed on the shaft when the flywheel key was sheared. You want the surface of the round shaft to be as smooth as possible.

  7. 7

    Polish the entire exposed area of the shaft with emery cloth. The surface should be a shiny metallic colour before you replace the flywheel key. The cleaner the shaft, the better the fit of the key into the flywheel.

  8. 8

    Use a small hammer to tap the new half-moon key into the machined slot. Be careful not to raise any ridges on the surface of the exposed key, as this will interfere with the fit into the flywheel's vertically machined slot. Use the mill file to dress down any metal interference that may have occurred from the key installation.

  9. 9

    Place a thin coating of grease over the exposed shaft and key.

  10. 10

    Replace the flywheel over the crankshaft. Pay particular attention to the vertically machined slot in the flywheel, and make certain it aligns with the top side of the key.

  11. 11

    Replace the spring dog clutch and the starter cover, in the reverse order from which they were removed.

  12. 12

    Test the engine by starting and running. Check for full power by engaging the engine in work, such as mowing or tilling.

Tips and warnings

  • Avoid hitting obstacles when operating an engine that is directly connected to a device like a mower blade or tiller tines. Belt-driven equipment allows for a "slip" to occur when you encounter such obstacles.

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