ORFS Fittings: O-Ring Face Seal Benefits and Limits
Jul 2nd 2026
ORFS fittings use an O-ring face seal to control leakage at the mating surface. The threads pull the connection together, while the O-ring seals at the flat face of the fitting.
This design is why ORFS connections are common where hydraulic systems need strong leakage control, vibration resistance, and serviceable assembly.
Thread Sizing
ORFS fittings use straight threads. The outside diameter is commonly identified with dash sizes, and the pitch is measured in threads per inch under the Unified Thread Standard family.
A dash size is often related to sixteenths of an inch. For example, a dash number multiplied by 1/16 gives a nominal outside diameter reference. Thread pitch, dash size, and face-seal geometry should all be checked before selecting a replacement.
Why ORFS Fittings Are Useful
The O-ring provides a mechanical face seal that can perform well under pressure and vibration. Because the seal is at the face instead of along a long thread path, ORFS fittings can reduce the chance of thread-related leakage.
They are also practical for maintenance because the O-ring can be replaced when the fitting body and sealing surfaces remain in good condition. ORFS fittings are available in many configurations, including tees, elbows, unions, adapters, and swivels.
Carbon steel and stainless steel versions are common. Material choice should follow the system pressure, fluid, temperature, and corrosion environment.
Disadvantages and Assembly Risks
The O-ring is the critical part of the connection. If it melts, hardens, erodes, falls out of the groove, or is pinched during assembly, the fitting can leak even if the threads are correct.
O-ring fallout can be a particular problem when a fitting is assembled in a difficult orientation. The seal has to stay seated in its groove until the faces meet.
Overtightening does not fix a missing or damaged O-ring. It can damage the fitting, deform the connection, and make the assembly unusable. Before tightening, inspect the groove, O-ring, face, thread, and mating part so the seal is made where the design intends.