Gas burner-heat exchanger assembly for an external combustion engine , in particular for Stirling engines, comprising a gas burner and a heat exchanger , the burner comprising: - a front wall defining a front side of the burner and forming a pass-through opening for the exchanger , - a rear wall defining a rear side of the burner and forming an opening to exhaust combustion gases, - a tubular side wall extended between the front wall and the rear wall and about a longitudinal axis (L) of the assembly , - a tubular diffuser wall arranged inside the side wall and extended between the front wall and the rear wall and about the longitudinal axis (L), the diffuser wall having a perforation for the passage of a gas mixture from an outer side of the diffuser wall to an inner side of the diffuser wall where the combustion takes place, - an annular distribution chamber formed between the side wall and the diffuser wall to distribute the gas mixture on the outer side of the diffuser wall , - a combustion chamber formed inside the diffuser wall and delimited on the rear side by the rear wall and which receives the heat exchanger , wherein the heat exchanger is formed by a tube assembly extended in the combustion chamber and intended to be passed through by a working fluid of the external combustion engine and having a heat exchange surface exposed in the combustion chamber , wherein a minimal diffuser-exchanger distance (C) between the diffuser wall and the corresponding heat exchange surface ranges from 20 mm to 40 mm, characterized in that a longitudinal height (A) of the perforated area of the diffuser wall is A = 90 mm, a longitudinal height (B) of the heat exchange surfaces inside the combustion chamber is B = 90 mm, and an external diameter (D) of the heat exchanger inside the combustion chamber is D = 183 mm. - a front wall defining a front side of the burner and forming a pass-through opening for the exchanger , - a rear wall defining a rear side of the burner and forming an opening to exhaust combustion gases, - a tubular side wall extended between the front wall and the rear wall and about a longitudinal axis (L) of the assembly , - a tubular diffuser wall arranged inside the side wall and extended between the front wall and the rear wall and about the longitudinal axis (L), the diffuser wall having a perforation for the passage of a gas mixture from an outer side of the diffuser wall to an inner side of the diffuser wall where the combustion takes place, - an annular distribution chamber formed between the side wall and the diffuser wall to distribute the gas mixture on the outer side of the diffuser wall , - a combustion chamber formed inside the diffuser wall and delimited on the rear side by the rear wall and which receives the heat exchanger , wherein the heat exchanger is formed by a tube assembly extended in the combustion chamber and intended to be passed through by a working fluid of the external combustion engine and having a heat exchange surface exposed in the combustion chamber , wherein a minimal diffuser-exchanger distance (C) between the diffuser wall and the corresponding heat exchange surface ranges from 20 mm to 40 mm, characterized in that a longitudinal height (A) of the perforated area of the diffuser wall is A = 90 mm, a longitudinal height (B) of the heat exchange surfaces inside the combustion chamber is B = 90 mm, and an external diameter (D) of the heat exchanger inside the combustion chamber is D = 183 mm. <EOS>