Completed Preventative maintenance. Oil\filter, air filter, battery test and inspect, inspect all electrical and fuel connections, c\o cabinet. While pulling spark plugs, found the rear cylinder plug was cross threaded or over tightened at the factory causing damage to cylinder head. After several attempts, I was able to remove spark plugs assess the damage, and take some photos for the warranty repair claim. I was able to put anti-cease on new spark plug and thread back into cylinder head to keep generator in operation until repair can be made.