high temperature nickel base alloy such as the one commercially available under the designation ams 5754 the three tubes may be disposed concentrically as shown in fig3 and then bent to the configuration shown in fig2 in which the flowpaths between the input end 21a and the output end 21b are nonlinear this results in a stem 21 having input and output ends 21a 21b respectively which are integrally connected if desired the three concentric tube structure of fig2 and 3 may be provided by suitably machining a single workpiece for each desired tube in this connection conventional straight through processing can be employed to provide the concentric tube structure shown in fig3 it is to be noted that such conventional processing can be done when the input end 21a and the output end 21b of the stem 21 are linear in this approach after the concentric tube structure has been obtained the machined stem 21 can then be bent to the particular shape needed such as the stem 21 shown in fig2 referring now to fig4 a portion of another form of the stem of the present invention is generally designated 50 in the stem 50 primary tube 52 and secondary tube 54 are disposed within outer structural tube 56 spacers 55 are provided to protect the inner tubes 52 and 54 from contact with the hot outer tube 56 the stem 50 is again suitable.