On September 11, 2000, at 0830 hours mountain standard time, an experimental Worgull Hornet, N707BD, experienced a loss of engine power and made a forced landing on Interstate 17 after takeoff from the Montezuma Airport, Camp Verde, Arizona. During the attempted landing, the airplane landed hard as the pilot was maneuvering to avoid obstacles. The airplane, owned and operated by the pilot under the provisions of 14 CFR Part 91, sustained substantial damage. The airline transport pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed for the personal flight. A flight plan had not been filed. The flight was originating at the time of the accident. In an interview with a Safety Board investigator, the pilot stated that a spark plug was found detached from the forward cylinder. He reported that the spark plug threads in the cylinder heads are made out of aluminum and the threaded portions of the spark plug barrels are made of steel. In the pilot's written statement to the Safety Board, he indicated that new spark plugs had been installed prior to the last flight, and that they were torqued to the manufacturer's recommendation of 110-inch pounds. He further indicated that there was deterioration of the threads inside of the cylinder head. He recommended a visual inspection of the spark plug threads and cylinder head threads during periodic spark plug removal. A Federal Aviation Administration inspector examined the airplane at the owner's facilities on September 22, 2000. He noted that the threads on both cylinders showed evidence of wear and appeared flattened.