An intershaft seal system disposed between an outer shaft and an inner shaft which are concentric and separately rotatable comprising: (a) a forward mating ring adjacent to a high pressure region; (b) an aft mating ring adjacent to a low pressure region; and (c) a piston ring concentrically aligned with a spacer ring, said forward mating ring and said aft mating ring being separately disposed about and separately rotatable from a said piston ring, said spacer ring rotatable with said forward mating ring and said aft mating ring; characterised by said forward mating ring includes a plurality of hydrodynamic grooves (43 or 44) when said aft mating ring includes a plurality of divergent flow grooves or said aft mating ring includes said plurality of hydrodynamic grooves (43 or 44) when said forward mating ring includes said divergent flow grooves, said hydrodynamic grooves (43 or 44) communicate a non-symmetric pressure profile (15 or 16) onto said piston ring, each said divergent flow groove includes a pair of grooves (27, 28) which intersect at an apex, said pair of grooves (27, 28) disposed substantially symmetric about said apex, said apex capable of receiving a fluid as said piston ring rotates with respect to said divergent flow grooves, said apex directs said fluid substantially equally into said pair of grooves (27, 28), said piston ring includes a plurality of channels which direct said fluid onto said aft mating ring, said fluid enters said channels along an inner diameter face of said piston ring or an outer diameter face of said piston ring, said divergent flow grooves produce a fluid pressure profile (57 or 58) which is substantially symmetric along an aft face or a forward face of said piston ring with respect to said apexes so as to minimize twist along said piston ring. (a) a forward mating ring adjacent to a high pressure region; (b) an aft mating ring adjacent to a low pressure region; and (c) a piston ring concentrically aligned with a spacer ring, said forward mating ring and said aft mating ring being separately disposed about and separately rotatable from a said piston ring, said spacer ring rotatable with said forward mating ring and said aft mating ring; characterised by said forward mating ring includes a plurality of hydrodynamic grooves (43 or 44) when said aft mating ring includes a plurality of divergent flow grooves or said aft mating ring includes said plurality of hydrodynamic grooves (43 or 44) when said forward mating ring includes said divergent flow grooves, said hydrodynamic grooves (43 or 44) communicate a non-symmetric pressure profile (15 or 16) onto said piston ring, each said divergent flow groove includes a pair of grooves (27, 28) which intersect at an apex, said pair of grooves (27, 28) disposed substantially symmetric about said apex, said apex capable of receiving a fluid as said piston ring rotates with respect to said divergent flow grooves, said apex directs said fluid substantially equally into said pair of grooves (27, 28), said piston ring includes a plurality of channels which direct said fluid onto said aft mating ring, said fluid enters said channels along an inner diameter face of said piston ring or an outer diameter face of said piston ring, said divergent flow grooves produce a fluid pressure profile (57 or 58) which is substantially symmetric along an aft face or a forward face of said piston ring with respect to said apexes so as to minimize twist along said piston ring. said forward mating ring includes a plurality of hydrodynamic grooves (43 or 44) when said aft mating ring includes a plurality of divergent flow grooves or said aft mating ring includes said plurality of hydrodynamic grooves (43 or 44) when said forward mating ring includes said divergent flow grooves, said hydrodynamic grooves (43 or 44) communicate a non-symmetric pressure profile (15 or 16) onto said piston ring, each said divergent flow groove includes a pair of grooves (27, 28) which intersect at an apex, said pair of grooves (27, 28) disposed substantially symmetric about said apex, said apex capable of receiving a fluid as said piston ring rotates with respect to said divergent flow grooves, said apex directs said fluid substantially equally into said pair of grooves (27, 28), said piston ring includes a plurality of channels which direct said fluid onto said aft mating ring, said fluid enters said channels along an inner diameter face of said piston ring or an outer diameter face of said piston ring, said divergent flow grooves produce a fluid pressure profile (57 or 58) which is substantially symmetric along an aft face or a forward face of said piston ring with respect to said apexes so as to minimize twist along said piston ring.