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 cylindrical surface which is substantially parallel 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; and - at least one annular gasket, which is provided with a central through hole having a predefined internal diameter, wherein said gasket is designed to be axially inserted around said shank to make seal between said head and said bore, wherein said plug is provided with at least one annular seat for accommodating said gasket, wherein said seat is arranged at a root connection between said head and said shank, and wherein said seat comprises: - a radial annular contact surface which is substantially flat in the radial direction, which is obtained on a portion of said head facing said shank and which is substantially perpendicular to said central rotation axis, wherein said radial annular contact surface is designed to be in contact with a radially oriented annular contact portion of said gasket; and - an axial annular contact surface which is substantially flat in the axial direction, which is obtained on a root portion of said shank at said radial annular contact surface and which is substantially parallel to said central rotation axis, wherein said axial annular contact surface has an internal diameter which is substantially equal to the internal diameter of said central through hole, and wherein said axial annular contact surface is designed to form an axial centering shoulder for said gasket, wherein said axial annular contact surface is provided with at least one circumferential groove having an internal diameter which is smaller than the internal diameter of said axial annular contact surface, the sealing device being characterized in that said circumferential groove is obtained on said axial annular contact surface at a joining portion between said radial annular contact surface and said axial annular contact surface. - at least one bolt-type plug, which is provided with a head and with a substantially cylindrical shank, wherein said shank has a threaded cylindrical surface which is substantially parallel 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; and - at least one annular gasket, which is provided with a central through hole having a predefined internal diameter, wherein said gasket is designed to be axially inserted around said shank to make seal between said head and said bore, wherein said plug is provided with at least one annular seat for accommodating said gasket, wherein said seat is arranged at a root connection between said head and said shank, and wherein said seat comprises: - a radial annular contact surface which is substantially flat in the radial direction, which is obtained on a portion of said head facing said shank and which is substantially perpendicular to said central rotation axis, wherein said radial annular contact surface is designed to be in contact with a radially oriented annular contact portion of said gasket; and - an axial annular contact surface which is substantially flat in the axial direction, which is obtained on a root portion of said shank at said radial annular contact surface and which is substantially parallel to said central rotation axis, wherein said axial annular contact surface has an internal diameter which is substantially equal to the internal diameter of said central through hole, and wherein said axial annular contact surface is designed to form an axial centering shoulder for said gasket, - a radial annular contact surface which is substantially flat in the radial direction, which is obtained on a portion of said head facing said shank and which is substantially perpendicular to said central rotation axis, wherein said radial annular contact surface is designed to be in contact with a radially oriented annular contact portion of said gasket; and - an axial annular contact surface which is substantially flat in the axial direction, which is obtained on a root portion of said shank at said radial annular contact surface and which is substantially parallel to said central rotation axis, wherein said axial annular contact surface has an internal diameter which is substantially equal to the internal diameter of said central through hole, and wherein said axial annular contact surface is designed to form an axial centering shoulder for said gasket, wherein said axial annular contact surface is provided with at least one circumferential groove having an internal diameter which is smaller than the internal diameter of said axial annular contact surface, the sealing device being characterized in that said circumferential groove is obtained on said axial annular contact surface at a joining portion between said radial annular contact surface and said axial annular contact surface.