joining zone obtained according to the invention is shown in fig1 a in a preferred embodiment of the invention the joining zone 12 is formed by two toric surfaces 13 and 14 the radial section of which extends through 45 u00b0 and is constituted by a circular arc the two toric surfaces 13 and 14 in this case have their radial section in the form of circular arcs 114 113 of equal angular extent and have an equal radius r1 and r2 the convex toric surface 14 is joined at one end to the inner surface 111 of the bore 11 of diameter di and at the other to the concave toric surface 13 which is joined at its second end to the inner surface of diameter di 39 of the reamed zone b it is apparent that the angular extents of the arcs 113 and 114 need not be equal in which case the radii r1 and r2 would also not be equal in addition the two concave and convex surfaces 13 and 14 need not have their radial section in the form of circular arcs even though the embodiment illustrated hereinafter relates to this particular case which is however non limiting with particular reference to fig2 and 3 these show the front tool support head of the reaming.