1308199 Seals for pistons A-T-O Inc 21 May 1970 [22 May 1969] 24648/70 Heading F2T A seal ring assembly 10, e.g. for a piston 13, comprises a first split-ring 16, e.g. of spring steel, which fits in a groove 15 of the piston 13, see also Fig. 1 (not shown), and a second splitring 17 of thermosetting plastics which fits in the groove 15, in contact with the first ring 16 and with the surface, e.g. of a cylinder 12. The rings 16, 17 are formed in an expanded position to maintain a sealing force in operation, the steel ring 16 being about 0À060-0À065 inch thick, while the plastics ring 17 is about 0À375- 0À380 inch thick and 0À485-0À495 inch in axial width. The plastics material may be nylon, possibly including about 25-30% glass fibres and 5% molybdenum disulphide as a filler.