What was found
SUSPECT COMPOSIT THRUST PLUGS WERE MISMANUFACTURED OR IMPROPERLY INSTALLED ALLOWING TRUNNION TO SHIFT. THIS CAUSED AN OUT-OF-BALANCE CONDITION. THE TAIL ROTOR HUB AND BLADE ASSEMBLY WAS DYNAMICALLY BALANCED ON SEVERAL OCCASIONS. EACH TIME THE TAIL ROTOR VIBRATION WOULD RETURN WITHIN A FEW FLIGHT HOURS. THE TAIL ROTOR ASSEMBLY WAS REMOVED FROM SERVICE DISASSEMBLED AND THE PROBLEM IDENTIFIED. UPON ASSEMBLY, BELL REQUIRES A .002 TO .004 THOUSANDTHS PINCH FIT. THIS ASSEMBLY LOST ALL PRELOAD AND DUE TO THRUST PLUG WIRE HAD A LOOSE FIT OF .004 THOUSANDTHS.