Hydraulic Manifold Machining: Cleanliness and Pressure Tests

A hydraulic manifold, also called a valve block, is a machined block of aluminium, steel or cast iron in which drilled passages connect valves, ports and actuators, replacing hoses and fittings between them. A supplier holds the valve interfaces to the ISO mounting-surface standards, removes the burrs where internal bores intersect, delivers the cleanliness level the customer specified, and states who performs the pressure test.

What is a hydraulic manifold or valve block?

The block holds the hydraulic circuit as drilled passages. Valves bolt to its faces or screw into its cavities, pump and tank lines enter through ports, and cross-drilled bores carry the oil between them. Compared with a circuit of pipes and hoses it has fewer leak points and a fixed geometry, and the price is a machining job in which almost everything that matters is hidden inside the part.

Three things separate it from an ordinary milled housing. The valve faces and cavities follow published interface standards. The internal bores meet each other, so burrs form where no tool can reach them. And the part is judged by what it releases into the oil as well as by its dimensions. Milling is the main process, on blocks cut from bar or plate and, for series parts, on castings machined to final size. The industry is described under hydraulics and fluid power.

Which standards define the valve interfaces of a manifold?

A valve from any maker fits a block only if the block carries the standard port pattern, locating holes and surface condition for that valve. ISO publishes one standard per valve family. The list below is current as of 2026-10-30.

Table 1. Valve interface standards for manifolds

Source: ISO catalogue pages of the standards named, accessed 2026-10-30.

Two points matter for drawings. First, ISO 7368 changed in 2025, so a drawing that cites "ISO 7368:2016" refers to a withdrawn edition and should be re-issued with the year checked. Second, ISO 4401 and ISO 7789 carry an open withdrawal proposal in their life cycle without a date, so confirm the status before you start a new design. The standards fix geometry only. The drawing still has to state the surface finish and flatness of the valve faces, with the values taken from the interface standard. See surface roughness Ra and Rz for how those callouts are read.

Which port threads and pressure ratings apply?

Ports for hose and pipe connections follow one of two families on European drawings: metric threads with O-ring sealing under ISO 6149, or G (ISO 228-1) threads under ISO 1179. The standards state different working-pressure limits.

Table 2. Port standards and the working pressures they state

Source: ISO 6149-1:2022 and ISO 1179-1:2013, ISO catalogue abstracts.

Both standards add that the permissible working pressure depends on port size, material, design and operating conditions. A block cannot simply inherit the 63 MPa figure: the wall around the port, the material and the load cycle decide. ISO 6149-1:2022 also notes that its Introduction gives recommendations for the ports and stud ends to be used in new designs, so check which family a drawing calls up before quoting, and ask the designer when the thread is only shown as "G 1/2".

Why is deburring cross-drilled bores the critical step?

Passages in a manifold are drilled from the outside and meet inside the block. Each intersection leaves a burr on the inner edge of the bore. A burr left there can break off under pressure pulses and travel with the oil into a valve spool or seat, which is the failure the cleanliness requirement exists to prevent.

Reaching those edges needs a process that works from inside the part: thermal, electrochemical, high-pressure water jet or abrasive-flow deburring are the usual choices, and the right one depends on the material, the bore diameters and the edge condition required. Each leaves its own residue, such as slag, electrolyte or abrasive grain, which the cleaning step has to remove afterwards. Deburring and cleaning are therefore one sequence and should be quoted together.

The drawing controls the result through the edge condition. ISO 13715:2017 gives the rules for indicating and dimensioning edges of undefined shape; where an edge needs a defined shape such as 1 × 45°, ISO 129-1 applies instead. A note such as "all intersecting bores free of burrs" with no edge value invites two readings, one at the supplier and one at incoming inspection. Dimension the edge, or name the inspection method, for example borescope inspection of listed intersections.

How is the cleanliness of a manifold specified and measured?

Four documents appear in hydraulic and automotive drawings, and they answer different questions.

Table 3. Documents used to specify cleanliness

Source: ISO catalogue (ISO 4406, ISO 18413, ISO 16232), BSI abstract of ISO 4406:2021, Fraunhofer IPA flyer 2026 (VDA 19.1), Clean Controlling (ISO 16232 and VDA 19.1).

ISO 4406 describes oil, not a block. For a manifold, the useful document is ISO 18413: the buyer and supplier agree the cleanliness requirement and the inspection method, and the inspection document records both. ISO 18413 covers particles only, so oil film, rust and fibres from rags are not inside it, and the drawing should name them separately if they matter. Automotive customers call up ISO 16232 or VDA 19.1 instead. Both define how particles are extracted from the part and analysed, and the lab report states the limit it was judged against.

Status as of 2026-10-30: ISO 4406:2021 and ISO 18413:2015 were each confirmed by ISO in 2026. ISO 16232:2018 is flagged for revision, with a draft expected, so cite the year.

Who performs the pressure test, and what must the drawing say?

No interface standard in Table 1 or Table 2 sets a production test pressure. The port standards state working-pressure limits, and the proof pressure, hold time, test medium and acceptance criterion come from the system designer. ISO 10771-1:2015 is a different test: a fatigue method for the pressure-containing envelope of a metal component under cyclic internal pressure, in which the user sets the test pressure. It qualifies a design and is not a delivery inspection.

Three arrangements occur. The supplier tests the bare block before shipping. The customer tests the finished assembly with valves fitted. Or both do, the supplier to catch leaking cross-bores early. Write the choice on the drawing or in the order, together with the pressure, hold time, medium and whether the report is required. The test medium and the cleaning step interact: a block pressure-tested after final cleaning has been wetted and handled again, so the order should fix the sequence and say at which stage the cleanliness check applies. Whether a given order includes cleaning and testing at the supplier is agreed before quotation.

What belongs in the request for quotation for a valve block?

A drawing and a STEP file are the minimum, and these entries make the quote comparable. The technical drawing checklist covers the general title block items.

• Material and condition: for example an aluminium alloy, steel or cast iron, with the certificate required. See aluminium alloys for CNC machining for the options.
• Valve interfaces by standard and year, and the general tolerance class, ISO 2768-m as standard in our network and tighter on faces and bores where the drawing says so.
• Port type and thread standard, ISO 6149 or ISO 1179, with the edition.
• Edge condition at bore intersections, per ISO 13715.
• Cleanliness document, limit, sampling method and the lab report required.
• Pressure test: who, pressure, hold time, medium, acceptance, report.
• Packaging of a cleaned block: sealed ports and protection in transit.
• Batch size, so that fixtures and programming for the CNC milling are priced against the right quantity.

Ask for deburring, cleaning and testing as separate lines in the quote. A single price for the finished block hides which of them the supplier has covered.

Frequently asked questions

What is the difference between a hydraulic manifold and a valve block?

In practice none. Both names describe a machined block with drilled passages that connects valves, ports and actuators in one body. "Manifold" is the common English term; "valve block" and the German "Hydraulikblock" are used for the same part.

Which standard defines the valve mounting surface on a manifold?

It depends on the valve type. ISO 4401:2005 covers four-port directional control valves, ISO 5781:2016 pressure-reducing, sequence, unloading, throttle and check valves, ISO 6264:1998 pressure-relief valves and ISO 6263:2013 compensated flow-control valves. Cartridge cavities are in ISO 7368:2025 (slip-in) and ISO 7789:2007 (screw-in).

What pressure can an ISO 6149 port carry?

ISO 6149-1:2022 allows working pressures up to 63 MPa (630 bar) with non-adjustable stud ends and up to 40 MPa (400 bar) with adjustable ones. The permissible pressure still depends on port size, material, design and operating conditions.

Does ISO 18413 say how clean a hydraulic manifold must be?

No. ISO 18413:2015 defines what the inspection document contains and gives guidance on extraction and analysis. The required cleanliness level comes from ISO 12669 and ISO/TR 10686 or from the customer's own specification.

Is ISO 16232 still the valid edition for component cleanliness?

ISO 16232:2018 is current and replaced the ten-part 2007 series, but ISO lists it as to be revised, with a draft ISO/DIS 16232 expected. Its German counterpart, VDA Band 19 Part 1, appeared in a 3rd revised edition in 2025.

Sources

1. ISO 4401:2005, Four-port directional control valves, Mounting surfaces (ISO catalogue)
2. ISO 5781:2016, Pressure-reducing, sequence, unloading, throttle and check valves, Mounting surfaces (ISO catalogue)
3. ISO 6264:1998, Pressure-relief valves, Mounting surfaces (ISO catalogue)
4. ISO 6263:2013, Compensated flow-control valves, Mounting surfaces (ISO catalogue)
5. ISO 7368:2025, Two-port slip-in cartridge valves, Cavities (ISO catalogue)
6. ISO 7789:2007, Two-, three- and four-port screw-in cartridge valves, Cavities (ISO catalogue)
7. ISO 5783:2019, Code for identification of valve mounting surfaces and cartridge valve cavities (ISO catalogue)
8. ISO 6149-1:2022, Ports with truncated housing for O-ring seal (ISO catalogue)
9. EVS, ISO 6149-1:2022, scope note on recommendations for new designs
10. ISO 1179-1:2013, Threaded ports with ISO 228-1 threads (ISO catalogue)
11. ISO 13715:2017, Edges of undefined shape, Indication and dimensioning (AFNOR)
12. ISO 4406:2021, Method for coding the level of contamination by solid particles (ISO catalogue)
13. BS ISO 4406:2021 abstract, code doubling per step (BSI)
14. ISO 18413:2015, Cleanliness of components, Inspection document (ISO catalogue)
15. ISO 16232:2018, Road vehicles, Cleanliness of components and systems (ISO catalogue)
16. Clean Controlling, ISO 16232 and its relation to VDA 19.1
17. Fraunhofer IPA, VDA Band 19 Teil 1, 3rd revised edition 2025 (course flyer 2026)
18. ISO 10771-1:2015, Fatigue pressure testing of metal pressure-containing envelopes (ISO catalogue)

Questions on a drawing? sales@ertas.engineering

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