A sealing device for bores, in particular bores obtained on a header portion of a heat exchanger, the sealing device comprising: - at least one bolt-type plug, which is provided with a head and with a substantially cylindrical shank, wherein said shank has a threaded side surface and a base surface which is substantially perpendicular to a central rotation axis of said plug, and wherein said plug is designed to be inserted into a corresponding bore in a threaded coupling; - at least one disk-shaped gasket having a predefined thickness, wherein said disk-shaped gasket is provided with a substantially flat seal surface and with a substantially flat contact surface, which is opposite to said seal surface; and - at least one pusher assembly (36, 38), which is interposed between said bolt-type plug and said disk-shaped gasket along said central rotation axis, the sealing device being characterized in that said pusher assembly (36, 38) comprises: - at least one disk-shaped anti-rotational pusher, which is provided with at least one first projection, having a first predefined thickness and comprising a pusher surface, and with at least one second projection, opposite said pusher surface and having a second predefined thickness, wherein said second projection has the shape of any three-dimensional figure which is not axially symmetric around said central rotation axis; and - at least one circumferentially threaded ring, having a predefined thickness which is smaller than the second predefined thickness of said second projection, wherein said ring is provided with at least one through opening having a shape that is geometrically similar to the shape of said second projection, wherein, in the assembled configuration of the sealing device, said second projection is inserted into said through opening in a mating configuration, so that said second projection projects through said opening to adhere to said base surface, whereas said pusher surface adheres to the contact surface of said gasket, so as to push said gasket into its sealing position in said bore. - at least one bolt-type plug, which is provided with a head and with a substantially cylindrical shank, wherein said shank has a threaded side surface and a base surface which is substantially perpendicular to a central rotation axis of said plug, and wherein said plug is designed to be inserted into a corresponding bore in a threaded coupling; - at least one disk-shaped gasket having a predefined thickness, wherein said disk-shaped gasket is provided with a substantially flat seal surface and with a substantially flat contact surface, which is opposite to said seal surface; and - at least one pusher assembly (36, 38), which is interposed between said bolt-type plug and said disk-shaped gasket along said central rotation axis, the sealing device being characterized in that said pusher assembly (36, 38) comprises: - at least one disk-shaped anti-rotational pusher, which is provided with at least one first projection, having a first predefined thickness and comprising a pusher surface, and with at least one second projection, opposite said pusher surface and having a second predefined thickness, wherein said second projection has the shape of any three-dimensional figure which is not axially symmetric around said central rotation axis; and - at least one circumferentially threaded ring, having a predefined thickness which is smaller than the second predefined thickness of said second projection, wherein said ring is provided with at least one through opening having a shape that is geometrically similar to the shape of said second projection, - at least one disk-shaped anti-rotational pusher, which is provided with at least one first projection, having a first predefined thickness and comprising a pusher surface, and with at least one second projection, opposite said pusher surface and having a second predefined thickness, wherein said second projection has the shape of any three-dimensional figure which is not axially symmetric around said central rotation axis; and - at least one circumferentially threaded ring, having a predefined thickness which is smaller than the second predefined thickness of said second projection, wherein said ring is provided with at least one through opening having a shape that is geometrically similar to the shape of said second projection, wherein, in the assembled configuration of the sealing device, said second projection is inserted into said through opening in a mating configuration, so that said second projection projects through said opening to adhere to said base surface, whereas said pusher surface adheres to the contact surface of said gasket, so as to push said gasket into its sealing position in said bore.