An arm assembly for an agricultural header includes an arm configured to rotate about a pivot axis relative to a frame of the agricultural header and configured to support a cutter bar assembly of the agricultural header. The arm assembly also includes a mounting bracket assembly configured to couple the arm to the frame, one or more fasteners configured to couple the arm to the mounting bracket assembly, and a plate configured to maintain a tension of the one or more fasteners with respect to the arm and the mounting bracket assembly.