Abstract: A sealing ring assembly for sealing a joint between two members (20, 22). An annular sealing ring 28 abuts an annular backup ring (38, 44) of substantially the same diameter. The annular backup ring is split to permit radial expansion and has a greater thickness on its outer periphery than on its inner periphery. When the joint being sealed (24) is under high pressure, the sealing ring urges the backup ring to expand radially to cover any gap (32) between the members being sealed, maintaining the seal, even under high temperature conditions, and preventing the sealing ring from extruding into the gap.