. . . . "More particularly, as shown in FIGS. 1 and 2, the sleeve 14 is formed of a ceramic material such as alumina or boron nitride (i.e. an inert ceramic) having a coefficient of expansion of about 7-8???10-6/ ??? C. The forward end of the sleeve is closed at 16 while a rear end of the sleeve is open at 24 and bounded or circumscribed by a flat annular end face 26 lying in a plane substantially normal t" .