A ‘worn-out’ spiral bevel gear and pinion set was submitted for examination and evaluation. This was a spiral bevel drive set with the gear attached to a differential. The assembled unit was driving a new, large, experimental farm tractor in normal plowing and tilling operations. The primary failure was associated with the 4820H NiMo alloy steel pinion, and thus the gear was not examined. The mode of failure was rolling contact fatigue, and the cause of failure improper engineering design. The pattern of continual overload was restricted to a specific concentrated area situated diagonally across the profile of the loaded side, which was consistent on every tooth.