Small Engines (Lawn Mowers, etc.): Craftsman 6.5 Clunks and Wont Start, air hammer, person job

Question
For the last couple of years my mower has had a loud clunk/banging noise, especially at start up.  It appears to be in sync with the blade rotation but the blade is not hitting anything.  I recently hit a buried steel house ground rod.  The mower immediately stopped.  It restarted but the clunking was significantly louder.  It restarted, ran for a short while and then died.  It won't restart.  It's about 10 years old.  Should I fix or replace?  Thanks  

Answer
Hi Robert,

The clunk might have been a loose flywheel or something internal like a worn connecting rod.  To repair or replace, depends on the condition of the mower and whether you want to invest time and expense into it.

If it quit right after striking an object, I could almost guarantee that the flywheel key is sheared.  The timing would then be off.  The flywheel is located under the recoil/cooling shroud. Its' bore is tapered and mates with the tapered crankshaft. The keyway in the flywheel and shaft keeps it from spinning on the shaft.  A nut secures it.  Use an impact wrench on the nut or a strap on the flywheel.  Don't put force on the cooling fins or they will break.  The recommended way to remove a flywheel is to use a puller.   An alternative, is to tap on the crankshaft top, with a brass hammer or use a punch tip on an air hammer.   The flywheel will need to be held up and supported or upward pressure applied with 2 large prybars. (A 2 person job.) Care is needed so the flywheel doesn't crack or bend.  Also be careful to not mushroom over
the crankshaft top.  Replace the key.  Install the flywheel after cleaning the tapers and re-tighten the nut to 65 ft/lbs.
There is also the possibility of a bent or twisted crankshaft depending on the severity of the impact but usually the key just shears.  

Let me know how you make out.
Michael