Small Engines (Lawn Mowers, etc.): B & S 31C707 on a Yard Machine 42 mower, head bolts, valve clearances

Question
Previous ?'s to Tom at this URl:
http://www.allexperts.com/user.cgi?m=6&catID=1746&qID=4821100

Exhaust lobe was worn down 3/8 inch. Have replaced Cam assembly,piston,pushrods,tappets, both valves,crank timing gear.  Now runs 30 seconds and pipe & Muffler bright rud and then shears flywheel key.
Ideas? thanks Dean  

Answer
You obviously have an OHV.  Briggs have changed there website...now I need the engine model, 31C707, the type number, xxxx, and the trim number, xx, to look up your engine.

What are the valve clearances?  Is a push rod bent? Rolling the push rods on a peiece of glass works well to check them.

Is the piston installed correctly?  Did you torque the rod and head bolts?

Did you line up the timing marks?  I've been told you can incorrectly time the engine but I have never had this issue.  I always rotate the crank and watch the valves operate a few revolutions and then re-check the timing marks are lined up.  

The engine is obviously running way too hot causing the muffler to turn red.  

Does the engine start easy?  Correct spark plug?
Let me know the engine type and trim so I can look up the parts list.
Eric