This paper discusses the makespan minimization of a production batch within a specific concurrent system, seru production system. A seru production system consists of multiple independent serus. A seru is a compact assembly origination in which products are assembled from-the-beginning-to-the-end without disruptions. One capability of a seru production system is its responsiveness. A performance measure used to evaluate a seru system’s responsiveness is the makespan of production batches assembled within the seru system. This study addresses the makespan minimization problem through an optimal seru loading policy. The problem is formulated as a min-max integer optimization model. An exact dimension-reduction Algorithm is developed to obtain the optimal allocation that minimizes the makespan. We show that the solution space increases very quickly. In contrast, our algorithm is efficient with a polynomial computational complexity of O(n2), where n is the total number of serus in a seru system. To verify the usefulness of the developed exact dimension-reduction algorithm, we compare it with a widely practiced greedy algorithm through experiments. We find that our optimal algorithm is robust in most cases and the greedy algorithm is efficient when variability in production efficiencies is high. This result can guide us to adopt different algorithms in different business environments. If the variability in production efficiencies is high, e.g., new employees and/or new products assembly, the greedy algorithm is efficient. For other cases, our optimal algorithm should be adopted to obtain the minimum makespan. We also extend the method to the application of a rotating seru.