FLEXITALLIC and NO-BLOW
Are the names that come to mind for fire rings. They are spiral wound chevron shaped spring steel, stainless I think. They have an asbestos like material between the steel. When the gasket is clamped, the edges of the chevron shaped material is pushed narrower, and compress the fiberous material.
They are used in aircraft piston engine exhaust pipes to the head, and used in high pressure steam lines like in power plants and ships. High pressure steam line gaskets have an additional guard ring aound the spiral part.
WARNING; SEA STORY; THIS AIN'T NO SHIT!
Having stood under a steam flange gasket of the traditional head gasket type when it blew out on a steam ship, I can safely assure you that a flexitallic type is much better! And that was a mere 450 PSI, 500 degree F steam line of about 5000 horsepower. (yes, Guy, I warmed up the line correctly and all that)
I had some custom made for the exhaust uptakes on Detroit Diesel 149s, they don't blow out. They conform to an irregular surface and temperature variations, and vibration better than anything else I have experience with. Increases in pressure tighten the seal because of the orientation of the chevron edges towards the pressure. Perhaps they are offered thin enough to conform to our head / deck clearance spec?
Scott E