STEVEN F
Either the new spark plug is TOO SMALL, or you destroyed the threads.
Spock (rhp)
what does "spitting off" mean?
XTX
the spark plug is the WRONG size === do not operate the mower until it is corrected ____ in the present condition you can ruin the motor ............
oil field trash
Not only could the plug be cross threaded but you may also have not tightened the plug enough. Finally you might have the wrong plug size.
Marduk
You may have cross threaded and destroyed the threads. If so, you need a helicoil. It is like a spring that replaces a worn thread. You might have to take it to a service repair place or talk to your local hardware store people about it.