The invention concerns a key device intended to hold a nut retainer in the radial position, when tightening the nut on an associated screw connection, characterized in that it comprises a handle (54) and a head (52) comprising a housing (56) into which a tab of the nut retainer is inserted, said tab being intended to be folded 90° on a body carrying the screw connection, after tightening of the nut, one face (58) of the head having geometry that matches a portion of the body carrying the screw connection that receives the nut, such that the end of the nut retainer, inserted into the head of the key, is held exactly perpendicular to the outer surface of said portion of the body when the nut is tightened.