Assembly of a first sheet, of a second sheet and of a fastening part, the first sheet including a polygonal opening, the second sheet including an orifice, the fastening part being designed to be inserted successively through the orifice in the second sheet and through the polygonal opening in the first sheet and to obtain, through movement of the portion-of-a-turn type with respect to the first and to the second sheet, a sheeton-sheet dismountable assembly, the fastening part comprising: - a centring cylindrical portion extending axially between a first end and a second end of the centring portion, - a group of at least two clipping tabs (21, 22, 23, 24) extending axially (x) from the first end, the clipping tabs (21, 22, 23, 24) carrying locking toes facing radially outwards, - a bearing portion, extending radially from the second end of the centring portion, - the part further including a group of at least two locking appendages extending radially from the first end of the centring portion, the bearing portion including at least two retaining zones extending radially towards the outside of the centring portion, each over at least one angular sector and located between the locking appendages, the clipping tabs (21, 22, 23, 24) being included in a cylindrical volume with the same axial direction as the centring cylindrical portion, the geometric contour of this cylindrical volume being obtained by superimposing the contour of the centring portion and the contour of the locking appendages, so that the tabs and the centring portion can be successively inserted through the orifice in the second sheet without undergoing deformation, and without limiting the axial progression of the part, the polygonal opening and the locking toes being further defined so that, when the locking appendages are then in the process of clearing the orifice in the second sheet, the clipping tabs (21, 22, 23, 24) are in a position of clearing the polygonal opening involving a lesser-amplitude deflection of the clipping tabs than at least one other value of deflection that might be imposed on the tabs by the polygonal opening if the group of clipping tabs were in another angular position with respect to the polygonal opening. - a centring cylindrical portion extending axially between a first end and a second end of the centring portion, - a group of at least two clipping tabs (21, 22, 23, 24) extending axially (x) from the first end, the clipping tabs (21, 22, 23, 24) carrying locking toes facing radially outwards, - a bearing portion, extending radially from the second end of the centring portion, - the part further including a group of at least two locking appendages extending radially from the first end of the centring portion, the bearing portion including at least two retaining zones extending radially towards the outside of the centring portion, each over at least one angular sector and located between the locking appendages, the clipping tabs (21, 22, 23, 24) being included in a cylindrical volume with the same axial direction as the centring cylindrical portion, the geometric contour of this cylindrical volume being obtained by superimposing the contour of the centring portion and the contour of the locking appendages, so that the tabs and the centring portion can be successively inserted through the orifice in the second sheet without undergoing deformation, and without limiting the axial progression of the part, the polygonal opening and the locking toes being further defined so that, when the locking appendages are then in the process of clearing the orifice in the second sheet, the clipping tabs (21, 22, 23, 24) are in a position of clearing the polygonal opening involving a lesser-amplitude deflection of the clipping tabs than at least one other value of deflection that might be imposed on the tabs by the polygonal opening if the group of clipping tabs were in another angular position with respect to the polygonal opening.