along the height of core 16 the center to center distance along a horizontal line between any two adjacent tubes is approximately one and one quarter inches the center to center distance along a vertical line between any pair of tubes both in alternate rows 70 or 72 is approximately three quarters inch accordingly the vertical distance between the center lines of adjacent rows 70 and 72 is approximately three eights inch as illustrated in fig7 and 8 core 16 of the present invention is constructed in accordance with a novel method prior to assembly of core 16 each of end plates 24 has a honeycombed configuration including a generally planar body portion 74 and a plurality of pockets 76 extending from a common side thereof each of pockets 76 includes a generally cylindrical side wall 78 integrally joined to body 74 and a bottom wall 80 fig8 accordingly each of pockets 76 defines an open end 82 generally coplanar with body 74 and a closed or abutment end 84 which serves as an abutment surface for tubes 26 core 16 is constructed by inserting end 86 of one of tubes 26 into each of pockets 76 in end wall 24a preferably tubes 26 are fully inserted into pocket 76 so that tube end 86 abuts surface 84 of bottom wall 80 after one of tubes 26 has been so inserted into each of pockets 76 the pockets and tubes located therein.