<p>Premature cracking was recurrently observed in forged 6061-T6 bicycle crank arms, occurring either in the as-manufactured condition or during tightening onto the bottom bracket axle. To elucidate the root causes, a multi-technique investigation combining fractography, metallography with quantitative image analysis, ICP-AES chemical analysis, hardness measurements, and billet characterization was conducted. Fractography revealed predominantly brittle intergranular fracture accompanied by frequent grain boundary pull-out of coarse Mg₂Si particles. Quantitative image analysis confirmed markedly higher areal fractions of coarse Mg₂Si in failed crank arms (0.14–0.23%) compared to reference parts (0.007%). Bulk chemistry indicated a shift from a slightly Mg-rich composition in the reference crank arms (Mg/Si ≈ 1.77) to excess-Si in failed ones (≈ 1.51–1.53), thereby favoring grain boundary precipitation. Failed parts also exhibited lower hardness (111–113 HV1) than references (122 HV1), consistent with solute depletion associated with coarse, non-hardening Mg₂Si and a reduced T6 response. These coarse particles were already present in the billets, indicating insufficient homogenization and persistence through subsequent processing. Overall, premature fracture is driven by coarse Mg₂Si at grain boundaries with excess-Si chemistry further promoting embrittlement. Failure prevention should prioritize improved billet homogenization and microstructural acceptance criteria for forged components specifying allowable areal fractions of coarse Mg₂Si. In addition, chemistry should be controlled toward stoichiometric or slightly Mg-rich compositions within AA6061 limits, coupled with robust quenching and controlled artificial aging.</p>