Struggling to understand the difference between imperial and metric ORFS fittings?
Imperial ORFS fittings follow inch-based SAE standards, while metric ORFS fittings use ISO metric threads. Both share sealing mechanisms, but differ in thread type, size standards, and market preference.
The choice between imperial and metric ORFS fittings affects system compatibility, supply chain efficiency, and maintenance across global applications. Let’s explore the details to help you make the right decision.
What are ORFS fittings?
O-ring Face Seal (ORFS) fittings are widely used in high-pressure hydraulic systems due to their leak-free design and straightforward assembly.
ORFS fittings use a flat face and an elastomeric O-ring to provide a reliable, leak-proof seal between male and female components, even under high pressure and vibration.
These fittings are specified in SAE J1453 and ISO 8434-3, depending on whether the system uses inch or metric thread standards. ORFS fittings are common in agriculture, construction, and industrial equipment. They are known for their reusability, vibration resistance, and reliable sealing.
Imperial ORFS Fittings?
| Tube outside diameter | Thread |
|---|---|
| 6 | 9/16-18 UNF |
| 8 | 5/8-18 UNF |
| 10 | 11/16-16 UN |
| 12 | 13/16-16 UN |
| 16 | 1-14 UNS |
| 20 | 1 3/16-12 UN |
| 22 | 1 5/16-12 UN |
| 25 | 1 7/16-12 UN |
| 30 | 1 11/16-12 UN |
| 38 | 2-12 UN |
a Thread per ANSI/ASME B1.1 Unified Inch Screw Threads (UN and UNF).
If your equipment originates from North America, you’re likely dealing with imperial ORFS fittings.
Imperial ORFS fittings follow SAE J1453 and use inch-based threads like UN/UNF, designed for compatibility with JIC and other SAE systems.
Imperial Tube Size
| Tube outside diameter | Limits of the outside diameter mm |
||
|---|---|---|---|
| in | mma | min. | max. |
| 1/4 | 6,35 | 6,25 | 6,45 |
| 5/16 | 7,94 | 7,84 | 8,04 |
| 3/8 | 9,52 | 9,42 | 9,62 |
| 1/2 | 12,7 | 12,6 | 12,8 |
| 5/8 | 15,88 | 15,78 | 15,98 |
| 3/4 | 19,05 | 18,95 | 19,15 |
| 7/8 | 22,23 | 22,13 | 22,33 |
| 1 | 25,4 | 25,3 | 25,5 |
| 1 1/4 | 31,75 | 31,6 | 31,9 |
| 1 1/2 | 38,1 | 37,95 | 38,25 |
These fittings typically:
- Use threads such as 9/16-18, 7/8-14, etc.
- Conform to SAE J1926-2 for port stud ends
- Are interchangeable with many U.S.-made hydraulic systems
Imperial ORFS fittings are commonly found in:
- U.S. and Canadian-built machinery
- Aerospace and military systems
- North American OEM hydraulic systems
Metric ORFS Fittings?
| Tube outside diametera | Thread | |
|---|---|---|
| mm | in | |
| 6 | 1/4 | M12 × 1,5 |
| 8 | 5/16 | M14 × 1,5 |
| 10 | 3/8 | M16 × 1,5 |
| 12 | 1/2 | M18 × 1,5 |
| 16 | 5/8 | M22 × 1,5 |
| 20 | 3/4 | M27 × 2 |
| 22 | 7/8 | M30 × 2c |
| 25 | 1 | M33 × 2 |
| 30 | 1 1/4 | M42 × 2 |
| 38 | 1 1/2 | M48 × 2 |
Metric ORFS fittings are dominant in European and international markets.
Metric ORFS fittings follow ISO 8434-3 standards and use ISO 261 metric threads, such as M18x1.5 or M30x2, to connect with ISO 6149-2 ports.
Metric Tube Size
| Tube outside diameter mm |
Limits of the outside diameter mm |
|
|---|---|---|
| min. | max. | |
| 6 | 5,9 | 6,1 |
| 8 | 7,9 | 8,1 |
| 10 | 9,9 | 10,1 |
| 12 | 11,9 | 12,1 |
| 16 | 15,9 | 16,1 |
| 20 | 19,9 | 20,1 |
| 22a | 21,9 | 22,1 |
| 25 | 24,9 | 25,1 |
| 30 | 29,85 | 30,15 |
| 38 | 37,85 | 38,15 |
Metric tubing shall be preferred.
a This size will be included in the revisions of ISO 6149-1 and ISO 6149-2.
Key identifiers of metric ORFS fittings:
- Threads and tube ODs measured in millimeters
- Designed to match with metric tubing systems
- Often used with ISO-specified hydraulic components
Metric ORFS fittings appear in:
- EU-manufactured equipment
- Global OEM hydraulic systems
- Applications with ISO 6149 or DIN 2353 ports
What’s the key difference between Imperial ORFS Fittings and Metric ORFS Fittings?
Many engineers confuse the two. But the difference is more than just thread type.
The core difference is the thread system: Imperial ORFS uses SAE/UN threads, while Metric ORFS uses ISO metric threads. They are not interchangeable.
| Feature | Imperial ORFS (SAE J1453-3) | Metric ORFS (SAE J1453-2 / ISO 8434-3) |
|---|---|---|
| Thread Standard | UNF / SAE J1926 | ISO 261 / ISO 6149 |
| Size Reference | Inches / Dash sizes | Millimeters |
| Port Stud Ends | SAE J1926-2 | ISO 6149-2 |
| Market | North America | Europe, Asia, International |
| Compatibility | SAE hydraulic systems | ISO/DIN metric systems |
At QC Hydraulics, we offer both types to serve global sourcing needs.
Advantages of using ORFS fittings?
ORFS fittings have surged in popularity—and for good reason.
Their biggest advantage is a reliable, leak-free connection thanks to the elastomeric O-ring seal design, especially under high vibration and pressure.
Here’s why we (and our customers) love ORFS fittings:
Leak-Free Sealing
The soft O-ring compresses between two flat faces, eliminating the risk of spiral leaks found in tapered threads.
Vibration Resistance
Great for construction and agricultural equipment that experiences constant motion.
Easy Assembly
No complex torque settings—just tighten until the O-ring seats.
Reusability
Fittings can be disassembled and reconnected multiple times without damaging sealing surfaces.
High Pressure Capability
Rated to up to 63 MPa (metric) or 41.3 MPa (imperial), depending on size:contentReference[oaicite:0]{index=0}.
These qualities make ORFS a top choice for OEMs in North America, Europe, and Asia alike.
Disadvantage of using ORFS fittings?
No system is perfect—and ORFS fittings come with a few caveats.
ORFS fittings require cleaner tube cuts and better alignment during installation to prevent O-ring damage.
Here’s what to keep in mind:
O-ring Vulnerability
Contamination or improper assembly can damage the O-ring, causing failure.
Cost
Slightly higher unit cost compared to simpler designs like BSP or NPT.
Not Backward Compatible
You can’t mix imperial and metric ORFS threads—they look similar, but won’t seal.
Assembly Space
ORFS fittings are often bulkier than 37° flare or compression fittings, needing more installation clearance.
As a result, many systems use ORFS only in specific, high-pressure, high-vibration zones—not system-wide.
FAQs
Q: Can I mix metric and imperial ORFS fittings?
No. They use different threads. Mixing them risks leaks or damage.
Q: What materials are ORFS fittings made of?
At QC Hydraulics, we manufacture in SS304, SS316L, and SS316Ti, all with EN10204 3.1 MTC.
Q: Which countries prefer metric ORFS?
Most of Europe and Asia use metric systems. The U.S. and Canada stick with imperial.
Q: Is the O-ring replaceable?
Yes. ORFS fittings are designed for easy disassembly and O-ring replacement.
Q: What standards cover ORFS fittings?
- Imperial: SAE J1453-3 + SAE J1926-2
- Metric: ISO 8434-3 + ISO 6149-2
Conclusion
ORFS fittings are the go-to solution for leak-proof, high-pressure connections. Choosing between imperial and metric standards depends on your system’s origin and global sourcing strategy.
QC Hydraulics is a leading manufacturer of stainless steel ORFS fittings for over 20 years. Please click here to download the catalog of ORFS fittings.