Connecting rod bolts are critical fasteners in heavy equipment diesel engines, where their structural integrity is paramount for operational reliability. This study investigates the root cause of a connecting rod bolt fracture in a heavy equipment diesel engine which have 936 Horsepower, which precipitated a catastrophic engine breakdown involving low power, overheating, and oil leakage. The failure analysis employed visual observation, macrographic examination, and Vickers micro-hardness testing to evaluate the fracture morphology and material properties. Hardness testing results revealed a core hardness of 347 HV, confirming that the bolt material (AISI 8640) retained a proper tempered martensite structure consistent with standard specifications, thereby ruling out material deficiency or thermal degradation. Macrographic analysis identified a progressive failure sequence involving two distinct mechanisms. The primary bolt failed via High Cycle Fatigue (HCF), evidenced by a flat fracture surface and ratchet marks at the periphery, indicative of reversed bending forces. The fracture of the primary bolt caused a loss of clamping force, leading to the instantaneous failure of the remaining bolts via ductile overload, characterized by fibrous topography and shear lips. The study concludes that the root cause of the failure was mechanical joint instability (loosening), which introduced fatal bending stresses and initiated the fatigue mechanism.