SAE J 1298 covers the recommended diagnostic port sizes for use in measuring hydraulic fluid temperature, pressure, flow, and for obtaining fluid samples. See SAE J 1502 for the detailed coupling specifications.
SAE J1298-2010 history
2017SAE J1298-2017 Connections for Fluid Power and General Use - Hydraulic Couplings - Diagnostic Port Sizes and Locations
2010SAE J1298-2010 Connections for Fluid Power and General Use - Hydraulic Couplings - Diagnostic Port Sizes and Locations
2008SAE J1298-2008 Connections for Fluid Power and General Use Hydrualic Couplings Diagnostic Port Sizes and Locations
1997SAE J1298-1997 Connections for Fluid Power and General Use Hydrualic Couplings Diagnostic Port Sizes and Locations
1988SAE J1298-1988 Hydraulic Systems Diagnostic Port Sizes and Locations@ Standard
1983SAE J1298-1983 Hydraulic Systems Diagnostic Port Sizes and Locations
1980SAE J1298-1980 Hydraulic Systems Diagnostic Port Sizes and Locations