On June 25, 1997, at 1243 hours Pacific daylight time, an Enstrom F-28-F helicopter, N99YC, was substantially damaged after a loss of power during a takeoff/climb at Riverside, California. The pilot and two passengers received minor injuries. Visual meteorological conditions prevailed for the local flight and no flight plan was filed. The flight was destined for Corona Airport, Corona, California. According to the pilot, the loss of engine power occurred less than 200 feet agl. He attempted an auto rotation and landed hard. Postaccident examination of the engine revealed that the intake valve rocker shaft migrated out of the shaft bosses of the cylinder head. The shaft, cover, and nuts were found; the washers were not. The pilot stated that there were internal star washer imprints on the cover plate that is commonly caused by the torque or tightening of the nut. The helicopter had 61.5 total hours since new and received one inspection at 50.9 hours since delivery to the owner/pilot. The owner/pilot changed the oil and filter prior to the formal 50-hour inspection by a private contractor. The inspection form revealed no specific inspection of the retainer plates are required except for a general condition inspection of the engine, accessories, and of the fluid systems for leaks.