3. Investigation Results 3-1. Appearance (Photos 1 and 2) - The inspected product was broken at the pipe section where the drive shaft passes. - There is some galling on the inner peripheral surface of the rear side of the engine at the fractured part, which appears to have occurred during mold release during the casting process. 3-2. Fracture surface analysis (Photos 3 to 8) - From the appearance of the fracture surface, it is assumed that the fracture started from the rear of the engine (Photo 3). The fracture surface around the starting point is corroded, and it is not possible to determine from the fracture surface whether the fracture occurred from the inner or outer circumference of the fracture (Photos 4 to 6). ・A ductile fracture surface is observed in the crack propagation area (Photos 7 and 8). .