The heat exchanger has oval tube (1) walls which are pressed against solid slide-in inserts (2) at least along the narrow sides (16), which are pressed inwards in undulated form. The inserts come in a U-shape whose side legs (6) bear against the tube walls and whose length exceeds the interval between tube connecting holes (7), so that an incision (8) demarcated by the legs comes to lie between the tube holes. The incision has flow-enhancing sides (9). The tubes are held apart by rings or discs themselves pierced in the tube hole area, the outermost tubes in the stack etc having coolant connections as opposed by cover plates on the tube at the opposite end of the stack. The walls of the tube are pressed to the inserts by means of tools with corrugated pressure edges.
