Accident Sequence
The aircraft was at approximately 2,500 feet mean sea level when the engine stopped. Attempts to restart the engine were unsuccessful. The flight instructor assumed control of the aircraft and attempted to land on a highway. During the landing attempt, the aircraft struck powerlines and a telephone pole before nosing over and coming to rest.
Engine Examination
The engine was removed from the aircraft and subjected to a teardown inspection. The examination revealed that the crankshaft gear teeth were sheared. The gear was a pressed-on type.
Maintenance History
Approximately 140 flight hours prior to the accident, a repair station had re-nitrided the crankshaft following regrinding operations. The pressed-on type gear was not removed prior to the re-nitriding process, as required. Specifications for the gear state that it should have a minimum Rockwell hardness of 30N-77 and that any temperature in excess of 300 degrees Fahrenheit would affect the hardness. When tested, the gear exhibited a Rockwell hardness of 30N-39. The temperature required for nitriding is in excess of 985 degrees Fahrenheit.