Small Engines (Lawn Mowers, etc.): Tecumseh 6.75 starting problem, thatching blade, keyway

Question
I have a self-propelled Craftsman mower w/a Tecumseh 6.75 h.p. engine, probably 1998.  The first time I used my mower this year, I used the de-thatching blade.  This put a lot of stress on my mower and I think I sheared the key sometime during this use.  The next time I went to use it, it wouldn't start, beckfired occasionally, and sometimes kicked back almost breaking my arm.  After I researched it, I realized it was most likely a sheared key and sure enough it was.  Now, I have gone through three keys trying to get it running.  I would re-install a new key, checked the smooth spinning of the shaft, fine.  As I pull the cord it sounds like it wants to start but can't.  And the keys are getting sheared before the mower even starts.  I would pull the cord freely, but as soon as the engine wants to start, that loud banging happens.  I think this is when the key is sheared.  The local mower shop guy thinks it is something like a bad ball bearing because it only happens when the compression builds up.  Any thoughts of where I should be looking and what for?  I appreciate any help at all.

Thank you

Answer
Check the blade bolt for tightness.  Once in a while the steel keyway in the blade adapter will shear.  This will cause the flywheel key to shear.

I would remove the blade and blade adapter.  Check the blade adapter and crankshaft keyway for wear or cracks.  

Is there a washer on the blade bolt?
Are you torquing the flywheel nut?

Let me know.
Eric