Heat exchanger for gas, in particular for the exhaust gas of an engine

The exchanger (1) has a casing (2) for allowing circulation of exhaust gas to exchange heat with a coolant. A coupler (6) and a duct are connected to a coolant circuit and integrated to a gas reservoir forming piece (7) that is coupled with an end of the casing. The reservoir forming piece integrates a lug (9) fixed to an engine block, and another coupler (4) is coupled with an exhaust gas circuit. The reservoir forming piece is made from stainless steel or aluminum.