Small Engines (Lawn Mowers, etc.): lawnmower, runout, plug hole

Question
how do I know if the crank is bent, or if it's just the blade or something else?
-------------------------
Followup To
Question -
when mowing lawn, I ran over a bolt. ...
Answer -
If the crank is bent, it is not unusual for a shop to refuse to straighten it.  I have straightened cranks before in the past but cannot tell you how long the engine lasted afterwards (never heard) and our shop would not offer any kind of guarantee.  If this is an expensive mower, you may want to consider replacing the crank or replacing the short block (depending on how much damage was done).  If it is an inexpensive mower, cut your losses and ask the shop if they will give you a better trade-in allowance and buy a new machine there.  Unfortunately, it is fairly easy, with the forces and speed involved, to destroy a perfectly new mower by hitting something large or stationary.  Hope this helps.

Answer
Remove the spark plug, keep the plug lead away from the spark plug hole and make sure the mower cannot start accidentally.  Then tip the mower, spark plug hole up, so you can rotate the engine over and do a visual inspection to see if you can see a bend.  If you can see something bent, you will have to verify what is actually bent and proceed accordingly.  If you can't see anything with your eyes, you would need to remove the blade and use a dial indicator to measure the axial runout of the crankshaft (a good shop can do this for you if you don't have the tools).  There may be a specific runout allowed but I would recommend there should be no more than 0.010" runout at the end of the crank.  Hope this helps.