On July 7, 1993, at 2215 central daylight time, the pilot of a Cessna 172G, N3631L, sustained serious injuries from a propeller strike at Gregg County Airport, Longview, Texas. After departing the runway the pilot parked the airplane on the taxiway to perform an inspection of a flat nose gear tire. During telephone interviews, conducted by the investigator in charge, with the pilot, and in a review of the enclosed written statements, the pilot reported that during the landing roll severe vibrations developed in the airplane. The pilot exited the runway onto the taxiway and advised the passengers to remain in the airplane while he investigated the source of the vibrations. He further stated that he pulled out the mixture, turned off the ignition switch, and put the keys in his pocket, prior to getting out to inspect the airplane. He reported that when he bent over to look at the nose gear strut, the propeller blade took a half turn and struck him in the head. A student pilot rated passenger reported in the enclosed written statement that the pilot had stopped the engine prior to exiting the airplane. A lineman at the fixed base operation reported in his enclosed statement that he was asked by a passenger to remove the key, which he found still in the magneto switch. The lineman further stated that the interior of the airplane was dark, but he had to turn the key a total of three clicks to the left before it came out of the magneto switch. Examination of the airplane by Federal Aviation inspectors did not reveal any discrepancies with the magneto system. The ignition switch was checked and the key could only be removed when placed in the "OFF" position. During disassembly of the nose gear assembly the mechanic reported that the wheel was extremely loose, the bearings could be heard rattling, the axle bolt was only finger tight, and the lower strut bolt was bent. He further stated that he "found the tube had a small split, probably from age." Numerous attempts were made by the investigator in charge to obtain the Pilot/Operator Report(NTSB Form 6120.1/2). The report has not been received.