The 2,873-hour flight instructor, who reported having accumulated 550-hours in the same make and model helicopter, reported that during a training flight, while practicing touchdown autorotations, the helicopter shuttered with a slight vibration through the airframe on climb out. The flight instructor added that the vibration stopped and he elected to continue the climb turning toward a downwind leg for the next autorotation. The helicopter shuttered again, this time much more violently, while on the downwind leg. The pilot stated on the NTSB Form 6120.1, Pilot/Operator Accident/Incident Report, that "I felt like the gearbox was locking up or something." The rated student pilot who was flying the helicopter at the time then declared "you've got the controls" to the instructor. The flight instructor immediately entered autorotation and the shutter stopped. In an attempt to regain powered flight the instructor then reapplied throttle which immediately created a violent vibration again and he decided to roll throttle completely off and continue with the emergency landing. The altitude at the onset was approximately 400 feet above ground level (agl). The flight instructor initiated a 180-degree turn autorotation to align the helicopter into the prevailing wind. The flight instructor also stated that the transfer of controls and attempt to regain powered flight, followed by the delay in entering the autorotational turn, created a low airspeed and low rotor RPM situation that made the successful landing not possible. The flight instructor stated that he "waited until the last second and pulled the collective up to the stops and prayed." The helicopter landed hard and managed to remain in the upright position. The flight instructor and the rated student pilot were not injured. An FAA inspector, who traveled to the site, noted that the spark plug and helicoil was broken out on the number two cylinder. Further inspection of the number two cylinder revealed three separate cracks in the cylinder wall. At the time of the accident, the winds were reported from 070 degrees at 10 knots, gusting to 15 knots.