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Assuming there is no serious damage, a new flywheel key should be all you need - about 25 or 30 cents. To confirm that this is all you need, replace the flywheel without the key but line up the two slots as they would be if a key were present. Tighten securely (but it doesn't need to be to the full torque as this is just a test). This should permit the mower to start and run normally but I would not recommend using the mower to actually cut grass until you replace the flywheel key.
To install the new one, insert the key into the slot in the flywheel first and then slip the entire affair onto the crankshaft (I like to use a bit of WD40 for protection as well). The flywheel should seat securely with no detectable free play - it should be on straight and not rock back and forth at all. If this is not the case, the key may be in upside-down or there may be something or some particles of dirt or metal blocking it. Replace the washers, dirt screen, etc., and then hand thread the flywheel nut as far as it will go. Tighten to the specified torque (typically, 30-33 ft-lbs).
Note: There may be a cupped washer between the nut/screen and flywheel. This must be installed cupped-side facing the flywheel or else you will be probably be replacing the flywheel key again very soon :-(.
WARNING: Do not install a hard steel key in place of the recommended flywheel key as you will lose the protection that the soft metal provides and the next incident may be the last... See the section: Why soft metal keys must be used.
Then, replace the shroud, fuel tank, etc. If head bolts had to be removed, it is probably a good idea to slightly loosen all of the head bolts and then retorque them to the proper value in the recommended sequence for your engine.