I've got an old school Toro Proline with a Suzuki 2-cycle 22040 bought new in 2001 and used only at my house. I know extreme overkill but back in the day I had one of these for a lawn business and was convinced that this was the best machine ever built.

Several weeks ago I hit a flagstone that had a corner tilting up and although it made a pretty good bang the engine never died. However shortly after that it started knocking and got progressively worse over the next couple of mowings and finally died. I tried to get the flywheel of and I couldn't so I had a local shop pull the wheel. I got a new key and put it back together and it ran for a 5 to 10 seconds (still knocking) and died again and the timing was way off. I pulled the wheel again and the key was sheered again (I did torque the flywheel to 36 lbs). Inspecting the inside of the flywheel the wheel is scored from where the key sheered and slid about a quarter inch. Although I can feel it, it doesn't seem that it is enough to sheer the key on it's own. I've inspected the piston for the exhaust port and it looks spotless. Same from the spark plug. I knew the blade had a little chunk removed by the stone but didn't view it as that big of an issued. The bolt retaining the blade for some reason was finger tight, but not sure if that would cause any problems. There is a little chuck (vertical) movement to the cam but nothing side to side.

I'm stumped as to what to do next. I've never seen or heard of the mower sheering a key without hitting something. Maybe the flywheel slot is rounded just enough to cause the key to sheer while just running.