A loss of power of the No. 1 engine for reasons that could not be determined, and the incorrect installation of a B-nut fitting in the propeller feathering system, which allowed engine oil to spray onto the exhaust system when the propeller was feathered following the loss of engine power. Contributing to the accident was an incorrectly repaired fuel leak, which resulted in an explosion that separated the aileron bell housing that resulted in a loss of control and subsequent impact with terrain.