Abstract: An improved louvered fin heat exchanger having plural fins arranged in substantially parallel, closely spaced relation and a plurality of heat transfer tubes passing through aligned openings in the fins is disclosed. Each fin includes a plurality of first and second louvers which intersect a nominal plane of the fin at respective first and second angles such that the first louvers are in diverging relationship with the second louvers on one side of the fin and in converging relationship with the second louvers on an opposite side of the fin, thereby providing a bi-directional louver configuration. Each fin further includes a plurality of collars defining the respective fin openings. Each collar has a substantially cylindrical portion and two distinct base portions on one side of the fin which defines two distinct recesses on an opposite side of the fin.