Because GB1235-76 is officially "replaced," any machine built after 2012 should use or ISO 3601-1:2018 . However, for legacy equipment, you have two choices:
During the 1970s, 80s, and 90s, China exported significant amounts of machinery to developing nations and utilized vast amounts of domestic equipment for infrastructure projects. This equipment—ranging from agricultural tractors to heavy hydraulic gates—was built with grooves designed explicitly for GB1235-76 O-rings. gb1235-76 pdf
Many Chinese hydraulic seal manufacturers (e.g., NOK-Freudenberg, TTO, or DSH Seals) have "legacy dimension" charts available for free download. While these are not the official standard, they contain identical dimension tables. Search "GB1235-76 O-ring groove chart PDF" on manufacturer support sites. Because GB1235-76 is officially "replaced