I have machined a lot of rings and seals rider rings from ptfe filled materials lots of different ones from white brown black gold colour contained bronze.lots of them contained graphite loft your hands black handling them.
if they were used with steam i don't know.i made rings in every form you can think of.
Plain straight cut ,angle cut ,steps . two piece or three piece which steps at each join i believe they had pins to locate the jion. the material changes size depending on temperature ,that would make it hard to work out what gap was needed.
i asked how these materials went with hard chrome and was told they didn't they only had one material they would use . they normally used a hard grade of carbide with a high rake for machining .the machine i used
mostly was the only one that used diamonds for turning.while the carbide is sharp the diamonds are not the sharpest cutting edges but they worked on the machine i used only one of the materials would wear them so they would not work on the normal materials again otherwise you would not think of changing tools for months at a time. John
they normally finished the bores with a hone just like an automotive engine .
