A hydraulic manifold drawing tells a machine shop where the passages go. It usually does not say how the part should fail-proof itself against cross-leaks, contamination, or a plug blowing out at pressure. Those gaps become problems later: a quote that looked cheap grows with change orders, or a block passes the bench test and starts leaking between circuits three months into service.
Below are the eight items we most often find missing from manifold RFQs. Putting them on the print, or in the RFQ notes, gives you comparable quotes and fewer surprises at first article.
1. Minimum remaining wall thickness between passages
Drawings show drilled passages at their nominal centerlines. Real deep holes drift, and drift grows with depth-to-diameter ratio. If two parallel passages are designed with a thin web between them, a small amount of drill walk can cut that web down to almost nothing.
What to specify: a minimum allowable wall thickness between any two passages (and between a passage and the outer face), not just nominal positions. Ask the supplier to confirm how they will verify it, for example by sectioning a first-article part or by CT scanning for critical blocks.
2. Cross-hole plug type
Process holes that make internal intersections possible have to be sealed afterward. Plug choice changes both cost and safety. Engineered expander plugs come with published pressure ratings and installation specs. Threaded plugs with sealant, and pressed balls, are cheaper but behave differently under pressure and vibration.
What to specify: the plug manufacturer and part number, or at least the plug style and required pressure rating. “PLUG PER NOTE 5” is only useful if Note 5 is actually complete. On aluminum blocks, also consider galvanic compatibility between plug and body material.
3. Cartridge valve cavity callouts and gauging
Cartridge valve cavities have tight form tolerances on their seal steps. They are cut with dedicated form tools that wear, and wear faster in ductile iron and steel than in aluminum.
What to specify: the exact cavity designation from the valve maker (for example the Sun Hydraulics or HydraForce cavity code), and that cavities must be checked with the matching cavity gauge at a defined frequency. A bore-diameter check alone does not prove a cavity is good.
4. Port standard, written out in full
“7/16-20 UNF” and “SAE -4 ORB” describe the same thread, but only the ORB callout (per SAE J1926) tells the shop it also needs the O-ring spot face and seal geometry. Mixing NPT, BSPP, and ORB ports on one block is common and is a frequent source of mistakes.
What to specify: the full port standard for every port, plus a port schedule table if there are more than a few.
5. Cleanliness as a number, not an adjective
“Clean and free of chips” is not a specification. Most U.S. buyers don’t put a cleanliness spec on manifold prints, but they still expect the block to arrive ready to run.
What to specify: a target code and method. For example, an ISO 4406 code for flushed fluid, or a gravimetric or particle-count limit under ISO 16232 for component cleanliness. Ask for the test report to ship with the parts.
6. Deburring method by material
Thermal energy deburring (TEM) is very effective on steel and ductile iron. It is not automatically right for aluminum, where thin edges and small holes are more sensitive to heat, and it leaves an oxide residue that needs follow-up cleaning.
What to specify: that internal intersections must be free of burrs and loose material, and ask the supplier to state their deburring and post-cleaning process for each material.
7. Coatings, with masking
Anodize and electroless nickel both add thickness, and both can coat bores and threads. If seal surfaces and cavities aren’t masked or compensated, the cartridge valve won’t seat. Also note that some chromate conversion finishes contain hexavalent chromium, which is restricted under RoHS and REACH.
What to specify: coating type and thickness, a masking callout for every cavity, port, and sealing face, and any RoHS or REACH requirement.
8. Pressure test method
A simple air-under-water test may pass a block that cross-leaks between adjacent circuits under hydraulic pressure.
What to specify: test medium, test pressure (often a multiple of rated pressure), hold time, and whether each circuit must be tested in isolation while neighboring circuits are monitored for leakage.
A one-page RFQ addendum
If your drawings are already released and you don’t want to revise them, attach a one-page addendum that covers all eight items. In our experience it is the cheapest way to make quotes from different suppliers comparable, and to keep the low bid from turning into the most expensive one.
If the block or its hardware runs in an ultra-high-strength alloy, the material callout needs the same discipline — see MP159 (AMS 5842) for what to specify when “MP159” alone isn’t a purchasable condition.
Oakmet reviews every manifold print for these items before quoting and flags anything missing. Send us your drawing and we’ll come back with a quote and a list of any open questions.