What might be a consequence of improper crankshaft balancing?

Improper crankshaft balancing can lead to excessive vibrations and noise in the engine. When a crankshaft is not properly balanced, the uneven distribution of mass causes it to oscillate as it rotates, which can introduce vibrations throughout the engine and vehicle. These vibrations can lead to discomfort for passengers, potential damage to engine mounts, and accelerated wear on engine components. Additionally, the increased noise generated by these vibrations might indicate problems that could worsen if not addressed.

The other options, such as cylinder head failure and enhanced durability, do not directly result from poor balancing. Increased engine efficiency is typically associated with harmonious operation and proper component alignment, which balancing aims to achieve. Thus, option C accurately reflects the consequences of improper crankshaft balancing.