mg and balance al it is important that the c shaped receiving grooves forming the interlock be arranged diametrically relative to the center axis passing through the cylinder formed by semi cylindrical cradles 8 and 9 in this manner unequal stress of the compound joint can be prevented the wall thickness of semi cylindrical cradles 8 and 9 can be thinner than the wall thickness of lower section 1 or upper section 7 at receiving grooves 3 and 4 or 5 and 6 because expansion is easier when a thinner wall thickness is used strength is maintained by rigid arrangement of the expanding tube in the semi cylindrical cradles fig5 shows an alternative embodiment of the present invention a spreader tube 11 is provided showing an oval cross section with a vertical axis perpendicular to the contact face 10 of the lower section this arrangement is useful because it is possible to interlock the upper and lower sections even with narrow upper and lower sections while spreader tube 11 is inserted between the two sections this embodiment is also useful for use with sections having wide interlocking grooves 12 15 cradle 20 is provided with an additional depression 21 which facilitates a vertical perpendicular arrangement of spreader tube 11 fig5 also shows an alternative arrangement of interlocking grooves 12 15 the c shaped grooves shown in fig1 4 have been replaced with e shaped.