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 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, 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 between the diffuser wall and the corresponding heat exchange surface ranges from 20 mm to 40 mm, characterized in that a longitudinal height of the perforated area of the diffuser wall is A = 90 mm, a longitudinal height of the heat exchange surfaces inside the combustion chamber is B = 90 mm, and an external diameter 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 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, 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 between the diffuser wall and the corresponding heat exchange surface ranges from 20 mm to 40 mm, characterized in that a longitudinal height of the perforated area of the diffuser wall is A = 90 mm, a longitudinal height of the heat exchange surfaces inside the combustion chamber is B = 90 mm, and an external diameter of the heat exchanger inside the combustion chamber is D = 183 mm.