1 are flat see fig2 at two diametrically opposed locations see fig2 there are tapered bolts 66 which secure the disc 35 to the ring 30 the heads of the bolts 66 are in recesses in the outer periphery of the ring 30 and the bolts pass radially through the ring and into the blind bores in the disc 38 this enables the structure 32 to pivot about the axis designated y this axis is indicated as a line in fig2 and is shown as a dot in each of fig1 and 3 as it is at right angles to the plane of the drawing internally of the casing 64 the structure 32 comprises a sleeve 70 within the sleeve 70 there are longitudinally extending ribs 72 and a tube 74 only shown in fig2 the drive shaft 22 passes through the tube 74 upper and lower bearings 76 78 for the shaft 22 are mounted at the upper and lower ends of the tube 74 the casing 64 and sleeve 70 extend downwardly to the region of the propeller 26 the space within the sleeve 70 is at its upper end connected to the exhaust port of the engine.