Abstract: A unistage drum (1) for building tires (2) , the drum (1) having two half-drums (13), each of which has a ring of sectors (43a) movable by a first actuator (37, 46) to and from an expanded position to clamp a respective bead (7); each sector (43a) being defined by a first and a second block (49, 50) defined externally by respective cylindrical surfaces (55a, 55) and having respective mutually-contacting inclined surfaces (52, 53); and the second block (50) sliding on the first (49), by virtue of a second actuator (33, 37), to and from an outer position in which the second block (50) defines a relative portion of an annular shoulder (56) located, in use, between the two beads (7) and in a position adjacent to one of the beads (7).