At about 0710 mountain time, a Stinson 108-1 lost engine power and nosed over in a field near Eloy during the ensuing forced landing. The pilot suffered only minor injuries. The pilot told investigators she was flying at 4,500 feet when she heard a bang, followed by a total loss of power. The engine was examined and the number 3 and 4 cylinders were badly damaged. The number 4 cylinder exhaust valve had fractured at the stem and was missing.