Small Engines (Lawn Mowers, etc.): crank, hub key, plug hole

Question
My wife hit a low lying tree stump with our lawn mower, which now has a mean vibration. Walk behind, small B and S engine. How do I go about straightening the crank? I believe that is what is wrong.

Answer
I really can't recommend straightening the crank.  Although I have done it before, I never got any feedback on how long the mower lasted afterwards.  You can check if it's bent by first removing the spark plug and moving the plug wire away from the plug hole (so it can't start).  Tip the mower with the plug hole up and rotate the engine over and observe the shaft.  If you can't see a bend, you can check it with a dial indicator.  I don't know what the maximum allowed runout would be but I would think that more than 0.010" (at the end of the shaft) would cause problems.  If the crank is not bent, you have likely sheared the flywheel and/or blade hub key(s).  These would need to be replaced.  Hope this helps.