outer pipe 21 are connected by way of connector means 22 in a threaded manner outer pipe 21 having a collar 23 also threaded thereto threads 24 between outer surface of pipe 20 and the inner surface of connector means 22 are tapered whereas threads 25 between the inner surface of pipe 21 and the outer surface of connector means 22 are straight threads 26 between the outer surface of pipe 21 and collar 23 are also tapered which is conventional for oil well pipe and collars in the normal straight thread 25 a shoulder 27 is left because of the way threads are conventionally cut such a shoulder is absent when using tapered threads 24 inner pipe 20 is subjected to high stresses the tapered threads 24 provide maximum strength in the connection and eliminate a shoulder such as 27 which can be a stress concentrator shoulder 27 is not a problem stress concentrator because of the tapered threads 26 the stresses between pipe 21 and collar 23 pass as shown by arrow 28 thereby bypassing shoulder 27 fig5 shows a cross section through the top part of two pipes first outer pipe 30 and second inner pipe 31 each have associated therewith a connector means i e first connector means 32 being associated with first pipe 30 and second connector means 33 being associated with second pipe 31 initially first connector means 32 is threaded.