THE HELICOPTER HAD LANDED AT A PRIVATE AIRSTRIP AND THE INSTRUCTOR DEPLANED. THE DUAL STUDENT REMAINED IN THE HELICOPTER WITH THE ENGINE RUNNING. THE CFI REBOARDED AND THE STUDENT MADE THE TAKEOFF. LATER, ON DOWNWIND IN TRAFFIC PATTERN, A WITNESS REPORTED A LOUD NOISE FROM THE ENGINE. THE MAIN ROTOR WAS OBSERVED TO SLOW NOTICEABLY AND THE HELICOPTER ENTERED A VERY STEEP DESCENT, IMPACTING THE GROUND ABOUT 1,000 FEET SOUTH OF THE RUNWAY. THE ENGINE ROD BOLTS ON THE NUMBER FOUR CYLINDER HAD FRACTURED. EXAMINATION OF THE FLIGHT CONTROLS REVEALED NO EVIDENCE OF PREIMPACT FAILURE. ONE OF THE ROD BOLTS WAS METALLURGICALLY EXAMINED AND SHOWED CRACKING THROUGH 25 PERCENT OF THE BOLT. THE FATIGUE WAS ATTRIBUTED TO INSUFFICIENT PRELOAD ON THE BOLT. THE ENGINE HAD 813 HRS SINCE A MAJOR OVERHAUL AND 36 HRS SINCE ANNUAL INSPECTION.