This report describes the nature of unusual bearing cap bolt fractures that occurred in heavy-duty industrial diesel engines. Electron fractographic examination provided the key element in the analysis of the fracture mechanism. Spontaneous rupture of the high-strength alloy steel bolts occurred as a result of stress-corrosion cracking. The aggressive environment necessary to develop this type of fracture was identified as water containing H2S. It was found that H2S was released from the engine oil additive, zinc dithiophosphate, and the necessary moisture was introduced during removal of the caps for bearing inspection after engine “run-in.” The development of these fractures required the occurrence of several events which accounts for their rarity.