Abstract: The present invention provides a valve assembly having a bore fitted with an operating member, whereby the operating member operatively controls a valve member to permit or prevent fluid flow through the valve assembly. The bore extends along a longitudinal axis and has a selected diameter in one portion and a diameter greater than the selected diameter on an interior portion. The bore further includes an annular transition portion extending with diminishing diameter from the interior portion to the one portion of the bore. The operating member is substantially cylindrical and includes a first portion which closely interfits with the one portion of the bore and a second portion which has an annular convex surface disposed to face the transition portion of the bore. The annular convex surface of the operating member and the transition portion of the bore form a metal-to-metal seat which performs desirably in a high temperature environment.