Advanced Finite Element Analysis (FEA) was performed to investigate the sealability of an API LTC connection under combined loads including tension, compression, internal pressure, and external pressure. The study showed two types of failure modes, Leak-Before-Yield under positive differential pressure (internal pressure higher than external pressure) and Yield-Before-Leak under negative differential pressure. Both failure modes exhibit a linear correlation with the Mean Normal Stress, defined as the average of the three normal stresses, but the slopes are different. The results raise a concern of potential connection leak significantly below several API ratings. In addition, connection dilatancy represented by the opening of thread root-crest space was quantified to assess the potential seal reduction from decreasing thread compound pressure resistance. Beneficial effect of Mean Normal Stress to mitigate connection dilatancy was also demonstrated.