On April 27, 1998, at 1500 central daylight time, a Robinson R22 Beta, N8331F, piloted by a commercial pilot, received minor damage during a forced landing in an open field following a tail rotor hub bolt failure while in cruise flight. Visual meteorological conditions prevailed at the time of the incident. The 14 CFR Part 91 photographic flight was not operating on a flight plan. The pilot and passenger reported no injuries. The flight departed Milwaukee, Wisconsin, exact time unknown. The pilot said the helicopter developed an abnormal vibration during flight. He said he reduced power and the helicopter yawed to the right a few degrees. He said he auto-rotated the helicopter with the engine producing partial power. He said he made a normal landing by adding power just before touch down. The on-scene investigation revealed that the tail rotor's hub bolt, part number NAS1304-38, had fully fractured at the point it passed through the tail rotor drive shaft. The hub assembly was attached to the tail rotor drive shaft by the fractured bolt. The self-locking nut was on the end of the fractured bolt. The manufacturer required secondary locking mechanism known as a palnut was not found. The torque stripe paint was cracked. According to the maintenance manual, "Any subsequent rotation of the nut on the bolt can be detected by visual inspection for cracked... torque stripes." A drawing of this assembly is appended to this report. The NTSB's Office of Research and Engineering Materials Laboratory examined the damaged bolt. According to the metallurgist's factual report, "...the fracture surface... from the separated hub bolt revealed that fatigue cracks emanated from multiple origins at the diameter surface... . These fatigue origins were located in the 12:00 and 5:00 o'clock positions, nearly on diametrically opposite sides of the fracture. One of the fatigue cracks propagated through approximately 60-percent of the bolt cross section and the other through approximately 20- percent of the bolt cross-section. The fracture outside the fatigue region exhibited features typical of overstress separation." The tail rotor hub bolt failed when N8331F had 889.2-hours on its airframe. N8331F had a 100-hour inspection on April 19, 1998, when the airframe had 865.3-hours on it. A review of N8331F's maintenance records showed that the tail rotor teeter bearings (bearings), part number B115-1, were replaced on February 21, 1998, when the airframe had 720.9-hours on it. The hub bolt passes through both bearings. The mechanic said he visually examined the bolt and believed it could be returned to service when he had installed the new bearings. He said he had torqued the bolt to manufacturer required 150-inch pounds when he replaced it. According to the R22 Beta's maintenance manual, the hub bolt is to be inspected anytime the hub assembly is taken apart. The maintenance manual's guidance for annual or 100-hour inspections instructs the mechanic to inspect the hub plate and hub. It does not provide any guidance to disassemble the hub and inspect the bolt. A copy of the manual's guidance is appended to this report. The maintenance manual requires that the tail rotor be re- balanced after parts have been replaced or work has been done on the assembly. The mechanic said he had not balanced the rotor blade assembly after installing the new bearings. He said he advised the owner to have it checked if they noticed a vibration. Excerpts from the helicopter's maintenance manual that address the alignment tool and balancing are appended to this report. The mechanic who had performed the last 100-hour inspection on N8331F and teeter bearing replacement was interviewed by a Federal Aviation Administration (FAA) Principal Maintenance Inspector. The mechanic said he did not use the helicopter manufacturer's required teeter bearing alignment tool when he had installed the new bearings on February 21, 1998. He said he did not have the alignment tool. The mechanic said he did not believe the tool was necessary because the bearing fit was not tight. A review of the FAA's Service Difficulty Reports showed no reports of tail rotor hub bolt failures for this model helicopter.