The pilot had just dropped off a load of logs and was en route back to the cutting area when he heard a loud bang concurrent with a left yaw and a rapid loss of rotor speed. The pilot entered an autorotation and collided with trees during the descent. Examination revealed that the main rotor drive shaft had failed. Examination of the shaft revealed evidence of a loss of grease from the shaft's boot, which resulted in an overheating condition and catastrophic failure of the unit. The 'O' ring, which provides the grease seal for the shaft boot, was found deformed with evidence that grease had been slung from the boot. The main rotor drive shaft had been removed for maintenance and replaced three days and 6 hours prior to the accident. The maintenance instructions call for a new 'O' ring to be installed when the drive shaft and boot are reassembled. According to the Bell laboratory, the 'O' ring appeared to have not been replaced.