ISO 5781:2016

Overview - ISO 5781:2016 (Hydraulic mounting surfaces)

ISO 5781:2016 specifies the dimensions, tolerances and port markings for mounting surfaces used with hydraulic pressure-reducing valves, sequence valves, unloading valves, throttle valves and check valves. The standard ensures mechanical interchangeability of valves and subplates used in industrial hydraulic equipment and manifold blocks, covering common port sizes and current industry practice.

Key topics and technical requirements

• Mounting surface dimensions: Defined for main port maximum diameters of 4.5 mm, 7.5 mm, 14.7 mm, 23.4 mm and 32 mm (codes e.g. 5781-02-01-0-16, 5781-03-04-0-16, etc.). Figures and tables specify exact layouts.
• Tolerances and surface quality:
  • Surface roughness per ISO 3601-2 requirements.
  • Surface flatness: 0.01 mm over 100 mm.
  • Locating pin hole tolerance: H12.
  • Position tolerances from origin: pin holes ±0.1 mm, screw holes ±0.1 mm, main ports ±0.2 mm.
• Fixing screw guidance:
  • Minimum thread depth 1.5 × D (screw diameter); recommended full thread depth 2D + 6 mm.
  • Recommended engagement for ferrous mountings 1.25 × D.
• Port marking and symbols: Port symbols and variants for directly-operated and pilot-operated valves are defined (tables for each port size). Note: the A→B direction is deprecated for new designs.
• Modular stack valves: For modular stacks, ISO 4401 mounting surfaces and markings apply.
• Rated pressure: Maximum working pressure should be specified by the supplier for subplates and manifold blocks.
• Identification statement: Standard provides a suggested conformity statement for catalogues and test reports.

Practical applications and who uses ISO 5781

ISO 5781 is used where consistent hydraulic valve mounting and interchangeability are critical:

• OEMs and system integrators designing hydraulic power units and machine controls.
• Manifold and subplate manufacturers producing valve mounting interfaces and blocks.
• Hydraulic valve manufacturers ensuring product compatibility across installations.
• Maintenance engineers and test labs verifying mounting dimensions, port markings and tolerances.
• Typical applications: industrial machinery, mobile hydraulics, presses, injection molding, and any system using pressure-reducing, sequence, unloading, throttle or check valves.

Related standards

• ISO 1219-1 - Graphical symbols for fluid power
• ISO 3601-2 - O‑ring housing dimensions/surface requirements
• ISO 4401 - Four-port directional control valve mounting surfaces (modular stacks)
• ISO 5598 - Fluid power vocabulary
• ISO 5783 - Code for identification of valve mounting surfaces and cartridge cavities

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