Abstract: The cam engine is a significant component of thermal power unit in underwater vehicles, and its performance is usually verified by power test. In a power test preparation for an underwater vehicle, some water leaked out between the bulkhead and driving gear of the cam engine. In order to determine the failure cause, three fault factors, including gaskets, personnel practice and set screws, were put into consideration and validation tests. Test results show that the fault is caused by excessive perpendicularity of set screws, which exceeds GB standards and caused seal failure. Then, increasing the perpendicularity of screws and improving inspection process of set screws are proposed. The research has actual effects on reliability design of set screws, and provides a necessary basis for perfecting relevant technologies of factory, army and so on.