On June 19, 1995, at 1440 central daylight time, a Bell 47J2A helicopter, N470RA, sustained substantial damage as a result of an engine failure and subsequent autorotation. The accident took place near Elkhorn, Wisconsin. The airline transport pilot and the passenger reported no injury. The 14 CFR Part 91 flight departed East Troy Airport, East Troy, Wisconsin, on a local flight. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported that he was cruising at about 1,000 feet above ground level when the engine failed. An autorotation was initiated. During the landing phase (cyclic flare), the main rotor struck the tailrotor drive shaft. An examination of the engine revealed that the number two cylinder intake valve had failed in the top radius (necked area) of the valve keeper recess, causing damage to the cylinder combustion chamber and piston. An inspection of the engine logbooks indicated that an identical failure had occurred eight hours previously. The first failure occurred on a factory new cylinder assembly, and the second on a "used" cylinder of undetermined time in service. Further inspection revealed that the major engine overhaul, which had been conducted 27 hours prior to the accident, had not been performed in accordance with the manufacturer's procedures and specifications. (See attachments)