Low drag 1275 piston standard size 2.780 plus 20 thou oversize(Rings included). This piston has a possible compression ratio of 10.3:1 as fitted to Cooper RSP 1990. The best performance 1275 pistons are C-STN100 range which have 9.73cr when the piston is 1mm down the block. . Current pistons differ from the 60s originals and now have a "low drag"…
3mm OIL RING with compression ring size .0591" (1.5mm)and one piece oil ring .1181" (3mm)on p21253 pistons giving 9.7cr on 1275cc. 20 thou oversize For performance see steel ring set with 3 piece oil ring as favoured by engine buildersC1818.020 Rover piston were stamped 21963 for high compression but 21962 seems to be stamped on some Metros pistons or overseas…
