A heat exchanger in which a heat exchanger core is disposed within a casing which includes a side wall formed by bending a plate into the form of a sleeve in which edges of the plate are in adjacent, confronting relationship with an inwardly projecting, transversely extending deformation, the deformation being disposed between a pair of press members with one of the press members being transversely inserted within the sleeve. The press members are moved together to remove the deformation with resultant pivoting of the edges into abutting contact or overlapping relationship. The heat exchanger core is formed of a plurality of stacked plate pairs each having a plate in inverted orientation. Outwardly projecting ribs of anticlastic form are so formed in each plate that the ribs on each plate of each plate pair are interengagingly intersect with the ribs on the adjacent plate of the adjacent plate pair accurately to align the plate pairs in the heat exchanger core.