SAE J1298 covers the recommended diagnostic port sizes for use in measuring hydraulic fluid temperature, pressure, flow, and for obtaining fluid samples. See SAE J1502 for the detailed coupling specifications.
SAE J1298-2008 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