from one 1 to three 3 final threads have a considerably reduced inside diameter causing an extreme interference with the male thread the penetration of the male thread exceeds the yield strength of the coupling 30 in the zone c coining the female thread material against the male thread profile and produces a true metal to metal seal thus in fig7 there is a stepped taper of the female thread but with substantially the same thread angle in each of the two sections of the taper in fig8 there are two different tapers with a larger thread angle on the section s in fig9 there is a stepped taper as in fig7 but with a larger thread angle in section s as in fig8 fig1 shows a fully made up joint in accordance with the coupling structure of any of fig7 9 inclusive and with the optionally included feature of a fully confined polymeric deformable sealing ring 50 disclosed in an annular groove 52 in the coupling substantially at the axial midpoint of the coupling thread referring now to fig1 and 12 and 13 fig1 shows in sectional view a partially made up joint where the female thread of the coupling is of slightly longer pitch than those of the standard api.