Repairing a Key on a Lawnmower Engine
Ever hit something hard with a lawnmower, and then it wouldn’t start? Most likely the cause is a little piece of metal, that holds the flywheel to the crankshaft, has been sheared.
The flywheel keeps the motor spinning between compression strokes, and holds a bit of energy. A sudden stop will cause this energy to be used up, usually in shearing a key. The key is made of a softer metal than the crankshaft, so the flywheel won’t damage the crankshaft. The flywheel also has the magnets that trigger the spark. When the flywheel spins enough to shear the key the magnet has moved past the proper timing point, and the engine won’t start (or stay running in a light hit).
The flywheel is usually on the top of a push mower, under the top cover, where the rope attaches. Removing the cover is usually as simple as removing 4 or 5 bolts around the top. The rope pull is contained in here, and will not come unwound if not damaged. There may be an oil dipstick attached to the cover, take care removing that so that grass and debris won’t fall into the oil.
At the top of the flywheel there will probably be a cup, with a single bolt attached to the crankshaft. This bolt will be very tight, and probably require using the blade to counter the wrench force. Using a 2×4, carefully block the blade in place so the blade won’t turn while loosening the top bolt. Pull with all your might to knock this bolt loose, or if an impact wrench is available, use that. Set the cup and nut aside.
The flywheel will still be attached firmly. The crankshaft has a taper to it, allowing the flywheel to grip firmly. There are special tools made for removing the flywheel, but what I have found works every time, is firm upward pressure with an oak board about 1 inch by 3/4inch by 30 inches long. Thread the nut back on about half way down the nuts threads (protecting the crankshaft thread). Then a firm tap on the nut on the crankshaft with a hard plastic mallet, one, maybe two taps will be all it takes, and the flywheel will be loose.
If there is a flywheel brake it may be necessary to pull that loose. Remove the flywheel now. The slot in the crankshaft will probably have half the key, and the flywheel will have the other half of the key. Carefully remove the parts that are in the slots, and clean out any metal chips in the area.
Your local small engine shop will have the proper key for the model motor you have. Put the new key in the crankshaft, and slide the flywheel back on lining up the slot for the key. The magnet may grab for the coil or other parts on the engine, be careful, it may cause
the flywheel to be misaligned. If there is a brake, it may be necessary to fiddle with that to get the flywheel back on.
Once the flywheel is back on, then put the cup and nut back on the crankshaft. Change the 2×4 that was blocking the blade so that it prevents the blade from turning while tightening. Again use all the force you have to get that nut tight. If the flywheel isn’t tight, normal engine running may shear the key. When you think you have everything tight enough, with the blade still blocked, try turning the flywheel. If the flywheel moves, tighten it more.
Replace the cover. Work the rope pull into the cup, so that it will turn the engine when pulled. Tighten the 4 or 5 bolts normal tight. If the dipstick came out, be sure to replace it carefully, trying not to knock any debris into the oil.
Take the mower outside, and give it a pull. It should start and sound normal.
- Remove the spark plug wire before starting the maintenance, to prevent acidental starting when putting the mower back together.
- Use gloves when hands are near the blade. Blocking the blade may cause contact with sharp sections.
- Clean all the debris out of the motor while it is apart. It will run cooler and last longer.
- Sharpen the blade. Whatever was hit will probably have nicked the blade. A sharp blade will have the mower doing less work.