a circumferentially extending groove 38 is machined in the outer surface of the neck portion 36 for receiving a snap ring 39 forming part of the protective sheath assembly 24 as best seen in fig4 the portion of the neck 36 below the groove 38 has a frustoconical profile defining a ramp 40 for facilitating the installation of the snap ring 39 in the groove 38 by sliding the ring 39 over the stem until the ring 39 captively falls into the groove 38 the snap ring 39 is made of a springy metallic material and is design to be received with a loose fit in the stem groove 38 as shown in fig2 the protective sheath assembly 24 further comprises an open ended tubular shield or sheath 42 adapted to be removably mounted to the fuel nozzle 22 so as to define an annular air gap about the stem 30 the sheath 42 and the annular air gap 44 see fig3 and 4 provide thermal insulation to the stem 30 in order to prevent the fuel flowing therethrough from coking the sheath 42 is preferably of unitary construction and is cylindrical in shape the sheath 42 has an inner circumferential wall 46 extending from a lower end 48 to.