Assembly (10) comprising a support (30) and a plate (20), in particular a support (30) for a watch movement and a plate (20) for a motor (31) of a watch part, as well as means for fixing the support (30) to the plate (20), the fixing means comprising: - a self-tapping screw (11) configured to form a thread when screwed in, the screw (11) comprising a head (12) and a longitudinal body (15), the body (15) comprising an end (19) opposite the head (12) and a thread (9) arranged around the body (15), the body having a
diameter that varies along the first axis of the body (15), being wider at the head (12) to a smaller
diameter at the end (19), - a first hole (16) passing through the support (30) or the plate (20), - a housing (21) arranged respectively in the plate (20) or the support (30) depending on the arrangement of the first hole (16),the housing (21) having a shape defined by a wall (8) and an opening (7) for receiving the body (15) of the screw (11), such that, the body (15) of the screw (11) passing through the first hole (16), the predefined shape of the housing (21) is threaded, when the screw is screwed into the housing, characterized in that the shape of the housing (21) has a variable width, which is wider at the opening (7) down to a smaller
diameter at the bottom (24) of the housing (21), the shape corresponding to that of the body (15) of the screw (11).