ISO 5781:2016 specifies the dimensions and other data relating to surfaces on which hydraulic pressure-reducing valves, sequence valves, unloading valves, throttle valves and check valves are mounted in order to ensure interchangeability.

It applies to mounting surfaces for hydraulic pressure-reducing valves, sequence valves, unloading valves, throttle valves and check valves, which represent current practice; they are generally applicable to industrial equipment.