Reference article — the questions customers actually ask, grouped by topic, with links to the full guides where each subject is covered in depth.

Standards and Interchange

Q: What is an ISO 7241 coupling? A: ISO 7241-1 is the international standard for hydraulic quick couplings of the ball-lock type, in two series: Series A (ISO 7241-1-A, the global interchange family used across Europe and Asia) and Series B (ISO 7241-1-B, more common in North America). Both are ball-lock designs; the two series are deliberately not interchangeable. The full comparison is in the ISO 7241 Series A vs Series B guide.

Q: What is the difference between ISO 7241, ISO 16028 and ISO 5675? A: Three different standards for three different jobs: ISO 7241 is the general-purpose ball-lock standard; ISO 16028 is the flat-face (no-spill) standard for high-pressure, low-contamination applications; ISO 5675 is the agricultural standard used on tractors and implements. See the standards comparison guide.

Q: Are hydraulic quick couplings interchangeable between manufacturers? A: Within the same standard and series, yes — that’s the point of the ISO standards. An ISO 7241 Series A nipple from one manufacturer mates with a Series A coupler from another, as long as the size matches. Across series or standards, no.

Q: What is a flat-face coupling and why does it cost more? A: A flat-face (ISO 16028) coupling seals on a flat mating face, so the sealing surfaces wipe clean on connection and the coupling is far more resistant to contamination and internal leakage. It costs more because of the precision flat lapping and the higher pressure rating.

Sizing and Selection

Q: How do I know what size coupling I need? A: Start with the circuit’s flow rate, then match the coupling’s bore to the hose ID, check the pressure rating against the system relief setting, and confirm the pressure drop at your flow is under ~1.0 bar. The five-input sequence is in the sizing guide.

Q: Can I use a coupling one size smaller than the hose? A: You can, but it becomes a bottleneck: pressure drop, heat, and reduced flow. The rule of thumb is the coupling bore should be at least the hose’s inner diameter. Size up when in doubt.

Q: Steel or brass — which should I choose? A: Steel (zinc-plated) for strength and most industrial circuits; stainless for corrosion and washdown; brass for lower-pressure, non-sparking and mild-corrosion applications. The full comparison is in the material guide.

Q: What pressure can a quick coupling handle? A: It depends on size, material and standard. A 1/2” steel ISO 7241 coupling is commonly rated ~250 bar (3600 PSI); flat-face couplings of the same size are often rated higher. Always use the manufacturer’s rated working pressure — never the burst figure — and keep it above your system’s relief setting.

Threads and Fittings

Q: What threads do hydraulic quick couplings use? A: The same thread families as the rest of the hydraulic system: NPT, BSPP (G), BSPT, JIC (SAE J514) and ORFS (SAE J1453). Identifying them is a 30-second job with a thread gauge — the identification table is in the thread standard guide.

Q: Can I connect an NPT coupling to a BSP fitting? A: Physically, sometimes — the pitches are close enough that they can start to thread. Sealing is another matter: NPT is a tapered pipe thread, BSPP is parallel, and the two do not seal together reliably. This is one of the most common (and most expensive) mistakes. Identify before you connect.

Q: What does “G” mean on a fitting? A: “G” is the common shorthand for BSPP (British Standard Pipe Parallel) thread, e.g. G1/2 is a 1/2” parallel BSP thread. A G thread seals on a washer or O-ring face, not on the thread itself.

Leaks, Stuck Couplings and Maintenance

Q: Why is my quick coupling leaking? A: The usual suspects, in order: worn O-rings and seals, contamination on the valve seat, incomplete engagement, a wrong mating pair, or a damaged valve face. The top-five breakdown is in the leaking coupling guide.

Q: My coupling drips when disconnected — is that normal? A: No. A drip when disconnected means the internal valve seal (poppet O-ring) is failing. It will also let air and dirt into the circuit, which damages the rest of the system. Reseal it — see the O-ring replacement guide.

Q: My coupling is stuck and won’t release. What do I do? A: Do not force it with a wrench or hammer. Depressurize the circuit, bleed the trapped pressure (use a relief tool on flat-face couplings), clean the nipple groove, and check the locking balls and sleeve. The ordered fix sequence is in the stuck coupling guide.

Q: Can I just replace the O-ring instead of the whole kit? A: You can, but the other seals are the same age and wear state — a partial reseal usually comes back within weeks. Seal kits are cheap; the labour of a second removal is not. Replace the kit.

Q: How often should I reseal my couplings? A: There is no universal interval. Reseal when a coupling leaks, as part of major overhauls, and on a 2–3 year schedule for machines that couple/decouple daily. See the maintenance guide.

Safety and Operation

Q: Can I disconnect a quick coupling while the line is pressurized? A: No. Depressurize the circuit first, every time. Residual pressure is what makes couplings pop, hoses whip, and oil inject. The safe connect/disconnect sequences are in the pressure safety guide.

Q: What is a pressure-relief tool and when do I need one? A: It’s a tool that threads onto a flat-face coupling’s relief port to bleed the trapped cavity before disconnecting. You need it whenever a flat-face coupling won’t release, and it’s the correct tool for breaking any stubborn joint. It’s covered in the pressure safety guide.

Q: What is an oil injection injury? A: A fine jet of hydraulic fluid at high pressure penetrating the skin. It looks minor but carries fluid deep into tissue and requires urgent surgical treatment. It’s prevented by never cracking pressurized connections and always bleeding first.

Q: Do dust caps really matter? A: Yes. Caps are the cheapest protection for the two most expensive failures: contamination on the valve seat (leaks) and dirt in the locking groove (stuck couplings). Fit them every time.

Sourcing and Supply

Q: Can I get quick couplings in mixed sizes and standards in one order? A: Yes — that’s the standard way to buy. Send the RFQ with the five inputs per line (size, standard/series, port thread, fluid, quantity) and the supply side can quote the full matrix. See the sizing guide for the checklist.

Q: What is the MOQ for standard ISO couplings? A: MOQ depends on size and standard — common ISO 7241 sizes are available from low quantities, while less common flat-face sizes or specialty materials carry higher MOQs. Ask on the RFQ; the answer is part of the quotation.

Q: How fast can standard couplings ship? A: Standard sizes and materials ship fastest — that’s the trade-off of standardization. The exact lead time depends on the mix and the destination, which is why the RFQ asks for the full size list up front.