John....
Remember the graphited yarn in this application will be a pressure energized ring....so yes by all means, machine the piston 'ring' groove as per Drawing dimensions and then as suggested unravel the yarn to add additional windings whilst still maintaining a vertical float for the yarn
The impregnated graphite provides excellent lubricity between the piston OD & the cylinder bore, and eliminates any potential for the natural twine/cotton/yarn to stick or burn at our modest steam pressure/temperatures and whilst at all times providing a ZERO steam bypass in your engine
People tend to dismiss 100 year old
technology just to use the latest & greatest ...[sometimes we must ask at what cost?]
[As a 1st year Apprentice
55+ years ago, I remember my Saturday overtime shift unravelling graphited yarn from those large Silver Boxes of J Walker Gaphited Packings, then repacking the gland seal of a 48" diameter [very scored] vertical piston rod in an oil/water extrusion press imported from the UK in 1948?......the actual gland seal was ten [10] split cut bands of 1" square JW Grade 600...[the 600 was a steam temperature rating in degrees F} and nothing to do with the oil/water fluid at 500 PSI working pressure @ ambient temperature]
So as that Apprentice, I asked my Tradesman why I could not make one semi continious ring of x 10 high instead of the 10 x individual staggered rings?......
&
... I then learnt a little of what amounts to fluid sealing characteristics as lectured by a Mr Daniel Bernouli ...which is part of the basis of thermo dynamics and still valid today some 300 years later
Derek