Nozzle for a blowing or stretch-blow molding of containers starting from parisons made of plastics, said nozzle having a nozzle body capable of coming in contact with an upper face of a mold designed for receiving a parison, said nozzle being characterized in that it comprises: An insert provided with a frusto-conical contact face capable of contacting a mouth of the parison, wherein said insert is displaceable with respect of the nozzle body between an upper position and a lower position, and A return spring which biases the insert towards its lower position.