Packing (10) for making a watertight seal between two threaded parts constituted by a truncated cone-shaped body made of TeflonR whose end of a smaller diameter is inserted in the female threaded connection while the end of larger diameter is inserted in the male threaded connection, screwing movement of the two threaded parts then causing the packing to modify its shape and fit into the two sets of threads said modification of its shape creating a secure and stable watertight seal.