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.

Ideally, I'd replace the flywheel. However, as previous poster suggests, check for bent crank first. If crank is OK, you can also try new key with old flywheel and increase torque on the nut. Given size of threads and that crank is steel and not cast iron, you should be able to go 50-55 ft/lbs., which will help keep things together.

I used to be run Suzuki's as well (22040 and 22043) and have parts, but my remaining short block is a zone start, and I believe yours is a BBC.