The military surplus helicopter experienced a 42 degree gear box failure while lifting a load of logs. The helicopter subsequently landed hard as the pilot was attempting to autorotate to a landing. Metallurgical examination of the gearbox revealed that a gear tooth on the input quill pinion had failed due to fatigue. No evidence of preexisting damage or improper overhaul procedures was found. The gearbox TSO was about 440 hours. The 42 degree gear box may be overhauled on condition. Investigation revealed that the helicopter had experienced in excess of 13,000 lift cycles in 380 hours since it was placed in service in a logging operation. The helicopter had been FAA-certificated in the restricted category for external load operations; however, Bell reports that the aircraft was not designed for repeated heavy lifting. There have been at least 2 other similar failures involving surplus UH-1/TH-1 helicopters used in logging operations. The landing site was strewn with cut timber and small trees.