the latter in a chamber of corresponding shape belonging to the casing 3 this ring 46 being secured to the intermediate disk 40 so that it rotates as one therewith by spacer pieces 47 similar to the spacer pieces 38 and therefore substantially axial profiled uniformly distributed in the circumferential direction about the axis x x and in an external radial position on the ring 46 as on the disk 42 this then yields an embodiment in which the exhaust diffuser 15 and the inlet nozzle 18 are substantially symmetric with one another with respect to the mid plane of the intermediate disk 40 perpendicular to the axis x x furthermore the widened head 39 of the intermediate disk 40 has in its external radial periphery a v shaped notch 48 so as to minimize stresses the face of the fixed casing 3 facing this having a corresponding v shape producing the compressor 2 plain radial exhaust diffuser 15 and the turbine 1 plain radial inlet nozzle 18 using the two annular rings 35 and 46 secured to the head 39 of the intermediate disk 40 which is free to rotate about the axis x x makes it possible for the diffuser 15 and the nozzle 18 to be arranged as a combined single unit free to rotate coaxially with respect to the turbine 1 and compressor 2 rotors 5 and 6 in other words.