A crankshaft has a fracture failure that appears to have initiated at a throw journal oil hole. Which of the following is a more likely cause?

The identification of a fracture failure in a crankshaft that originates from a throw journal oil hole suggests a significant issue related to the crankshaft's operation and supporting components. The failure in this scenario is most likely due to a failed vibration damper.

A vibration damper plays a crucial role in controlling the torsional vibrations produced by the engine during operation. If the damper fails, it can lead to excessive harmonic vibrations, which may stress the crankshaft beyond its designed limits. This excessive stress could initiate cracks or fractures at weak points, such as the oil hole in the throw journal, leading to the observed failure.

In contrast, while issues like a broken main cap, excessive cylinder pressure, or a spun rod bearing can also affect the crankshaft's integrity, they do not typically initiate fractures at oil holes in the same manner as a failed vibration damper. A broken main cap commonly results in misalignment or binding issues, excessive cylinder pressure tends to cause overall failure in engine components rather than localized fractures, and a spun rod bearing would typically lead to bearing damage rather than initiating a crack at the oil hole due to vibrations. Thus, the failure of the vibration damper represents the most plausible cause for the observed fracture failure in this context.