A manufacturing method of multi-pipe monolithic hydraulic devices, in particular heat exchangers, comprising the steps of: i) - producing a plurality of ducted metallic plates (14, 15) delimited by opposite flat faces and provided on at least one of said flat faces with one or more grooves or channels which extend between first opposite ends of the plates and parallel to second opposite ends (20, 21) of the plates arranged transversely to the first ends, leaving the second ends of the metallic plates solid and without grooves or channels; ii)- stacking the ducted metallic plates (14, 15) with the opposite flat faces rested directly one against the other to form a stack; and iii)- subjecting the stack of ducted metallic plates to at least one thermomechanical integration cycle by diffusion bonding so as to obtain, from the metallic plates, a single monolithic block within which the grooves or channels form a plurality of pipes realizing at least one hydraulic circuit (4; 5); characterised in that iv)- said plurality of ducted metallic plates (14, 15) have the same shape but are made two-by-two with a width, measured between said second ends (20, 21) of the same, progressively decreasing, starting from at least a first plate, or a first couple of plates having an identical width, arranged at a median plane (α) of the stack, the plurality of ducted metallic plates being arranged alternatively and symmetrically above and below the first plate or the first couple of plates; the width of the metallic plates (14, 15) being made so that the monolithic block that is obtained at the end of step iii) of diffusion bonding has a shape that is substantially a solid of revolution; v)- at the end of step iii) of integrating, by diffusion bonding, the metallic plates (14, 15), winding a metallic cable upon the monolithic metallic block with a shape of a solid of revolution obtained from the metallic plates, the metallic cable being tightly wound in a spiral and/or with a cross-winding on a lateral outer surface of the monolithic metallic block and then being firmly fixed to the latter. i) - producing a plurality of ducted metallic plates (14, 15) delimited by opposite flat faces and provided on at least one of said flat faces with one or more grooves or channels which extend between first opposite ends of the plates and parallel to second opposite ends (20, 21) of the plates arranged transversely to the first ends, leaving the second ends of the metallic plates solid and without grooves or channels; ii)- stacking the ducted metallic plates (14, 15) with the opposite flat faces rested directly one against the other to form a stack; and iii)- subjecting the stack of ducted metallic plates to at least one thermomechanical integration cycle by diffusion bonding so as to obtain, from the metallic plates, a single monolithic block within which the grooves or channels form a plurality of pipes realizing at least one hydraulic circuit (4; 5); characterised in that iv)- said plurality of ducted metallic plates (14, 15) have the same shape but are made two-by-two with a width, measured between said second ends (20, 21) of the same, progressively decreasing, starting from at least a first plate, or a first couple of plates having an identical width, arranged at a median plane (α) of the stack, the plurality of ducted metallic plates being arranged alternatively and symmetrically above and below the first plate or the first couple of plates; the width of the metallic plates (14, 15) being made so that the monolithic block that is obtained at the end of step iii) of diffusion bonding has a shape that is substantially a solid of revolution; v)- at the end of step iii) of integrating, by diffusion bonding, the metallic plates (14, 15), winding a metallic cable upon the monolithic metallic block with a shape of a solid of revolution obtained from the metallic plates, the metallic cable being tightly wound in a spiral and/or with a cross-winding on a lateral outer surface of the monolithic metallic block and then being firmly fixed to the latter.