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I've went over to Viton rings a few years ago and find they work very wellJust watch your groove sizes as for model use you don't want to compress the rings as much as most makers published data as we are not running at 20bar and the more the ring is compressed the more friction you will have to overcome.And on the subject of friction you will only need one ring on each piston not a pair.As an aside do the build articles mention why the small relief is cut into the mating faces of the piston? I can't see much point in it being there at such a shallow depth, Could be taken out deeper to lighten the piston as many a full size one was either in two parts or a core used to cast single piece ones hollow
SNIPSorry for all the questions !Kind regardsDerek