end to the downstream end of the disk 1 that is to say from left to right in fig1 hereinafter the term top respectively bottom will mean radially outer respectively inner the spacer 7 illustrated in fig2 and 4 is a single piece of hollow profiled shape it comprises a top wall 10 a bottom wall 11 and two side walls 12 a and 12 b the central part 40 of the profiled shape may where necessary contain a core not shown forming a whole with the spacer 7 preferably the top wall 10 is slightly domed the convex side being situated radially toward the outside that is to say toward the root 4 of the blade 3 the concave side being situated radially toward the inside of the spacer 7 its thickness may be constant or may vary along the axial and or tangential directions this top wall 10 is directly in contact with the bottom bearing surface of the root 4 of the blade 3 the bottom wall 11 comprises a central zone 15 and two peripheral zones 16 a and 16 b the peripheral zones 16 a and 16 b are in contact with the bottom 6 of the slot 2 the shape of which peripheral zones closely matches that of the bottom 6 the central part 15 is recessed toward the interior of the spacer 7 that is to say that the radial positions of the peripheral zones 16 a and 16 b on the one hand and of the central zone 15 on the other hand are different the central zone being further away from the axis of rotation of the disk 1 this disposition gives the whole spacer 7 flexibility and contributes to its spring effect the respective thicknesses of the three zones comprising.