We address the simple assembly line balancing problem: minimize number of stations m for processing n partially ordered operations V={1,2,...,n} within the cycle time c. The processing time ti of operation i of V and cycle time c are given. However, during the life cycle of the assembly line the values ti are definitively fixed only for the subset of automated operation of V-VM. Another subset VM of V includes manual operations, for which it is impossible to fix exact processing times during the whole life time of the assembly line. If j belongs to VM, then operation time tj can be different for different cycles of the production process. For the optimal balance b of paced assembly line with vector t=(t1,...,tn) of the operation times, we investigate the stability of its optimality with respect to possible variations of the processing times tj of the manual operations. In particular, we derive necessary and sufficient conditions when the optimality of the line balance b is stable with respect to sufficient small variations of the tj. We show how to compute the maximal value of independent variations, which definitively keep the feasibility and optimality of the line balance b.