The invention relates to a crank (14) which is intended for the windscreen wiper mechanism of a motor vehicle. The inventive crank is of the type that comprises a horizontal body (16) having a first end (14a) which is linked in rotation to a wiper arm and a second end (14b) which is articulated to a connecting rod. The aforementioned body (16) comprises: a segment (22) which can be deformed in order to adjust the distance between the ends (14a, 14b) of the crank (14), and a longitudinal oblong hole (24) which divides the aforementioned segment (22) into two deformable longitudinal branches (26) which are symmetrical in relation to a center line of said segment (22). The invention is characterized in that the oblong hole (24) takes the form of a quadrilateral with two essentially-perpendicular diagonals (E1, E2), the larger diagonal (E1) extending along the main axis (C) of symmetry of the segment (22) and intersecting the small diagonal (E2) at the midpoint thereof.