Anomalous Fractures of Diesel Engine Bearing Cap Bolts

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.