This part of ISO 6624 specifies the essential dimensional features of half keystone rings made of cast iron, types HK and HKB, having diameters of from 38 mm up to and including 160 mm, used in reciprocating internal combustion piston engines.
ISO 6624-2:2003 history
2016ISO 6624-2:2016 Internal combustion engines - Piston rings - Part 2: Half keystone rings made of cast iron
2003ISO 6624-2:2003 Internal combustion engines - Piston rings - Part 2: Half keystone rings made of cast iron