Abstract: The invention is directed to a method for improving the mechanical behavior of a metallic body (4) comprising an internal volume for a fluid and at least one threaded connecting port (6, 8) to said internal volume, the method comprising a step of treatment by autofrettage of the internal volume by applying a pressure to a liquid inside said volume. The autofrettage step comprises closing the internal volume by screwing a plug (28) to each the at least one threaded connecting port (8), so that the thread(s) of said port(s) is/are also subject to the autofrettage treatment. The invention is also directed to a body (4) resulting from such a treatment, with compressive stresses at the root of one of the most carrying turns of the thread of each of the connecting ports. The compressive stresses improve the fatigue behavior of the body.