cover 6 at a downstream end thereof this labyrinth seal 32 serves to limit the drop of air pressure in the air enclosure 20 in the event of carbon segments constituting the gasket 22 being destroyed with reference to fig2 a and 3 to 6 there follows a description of various embodiments of the clip for axially blocking the rim 26 inside the cover 6 in the embodiment of fig2 the axial blocking clip 30 comprises a resilient pin 36 that is received radially inside the rim 26 this resilient pin 36 is in the form of half an annulus centered on the longitudinal axis x x of the engine and when in position it is received inside an annular groove 37 centered on the axis x x and formed in the collar 26 b of the rim fig1 at equal distances from its two free ends 36 a and 36 b the resilient pin is provided with a latch 38 that when in position is to pass radially through the rim and more precisely its collar 26 b and be received radially inside an annular groove 40 formed in the cover 6 all around its circumference in a variant and as shown in part in fig2 a the latch 38 of the resilient pin can pass radially through the rim and be received inside a notch 40 u2032 formed in the cover 6 furthermore means are provided for locking the resilient pin 36 in position inside the rim 6 thus in the embodiment of fig2 the axial blocking clip 30 has two catches 42 each secured to a free end 36 a 36 b of the resilient pin 36 and each being received when in position.