Abstract: The broaching and free running of torpedo might occur in test, which result in the damage of the external-combustion engine cam.This paper analyzes the engine cam damages due to torpedo broaching in lake or sea test and engine free running in stand test on land, and calculates the force and stress on the cam during the free running. When rotational speed of cam is three times higher than the rated one, the contact stress increases rapidly to two times of the normal stress, i.e. 21 617kg/cm2, which obviously exceeds the allowable value (18 000 kg/cm2). The anatomic, matallographic, and crack analyses of the damaged cam caused by free running are conducted, and it is found that the cam crack is due to fatigue under overload. The conclusion is drawn that the deformed cam after free running can no longer be used.