An expansion tank for a motor-vehicle cooling system has an inlet port and an outlet port and is provided with a plastic shell having a container and two protruding collars, respectively at the two ports; at least one of the collars defines a seat, which is engaged by a tubular insert coaxial to the collar; elastically deformable teeth are provided for snap-coupling the tubular insert and the shell to each other and axially retaining the tubular insert in the seat.