lumen at the downstream tip of each element 52 referring to fig2 through 5 during assembly of improved extrusion head 10 u2032 the lumen pipe assemblies are first positioned on either of faces 19 a or 19 b with pipe elements 52 extending into die element 18 b in approximately the correct final position die elements 18 a 18 b are then joined assembly elements 50 remaining both radially translatable and rotatable therebetween the o rings 48 and pin vises 44 are then passed over elements 50 and the pin vises are threaded into the die the correct location and orientation of the portions of pipe assemblies 46 extending into the die are established through use of a dedicated gauge block 56 having a shape and size closely matching the extrudate shape of die portion 24 b u2032 in the example shown in fig5 gauge block 56 is substantially cylindrical block 56 is formed to be easily and close fittingly insertable into portion 24 b u2032 and is provided with precisely machine axiaily extending channels 58 on the outer surface thereof for receiving and positioning the lumen pipes precisely within portion 24 b u2032 the bottoms of channels 58 define the radial positions of the lumen pipes and the channels themselves precisely align the pipes with the axis of die element.