Posted on July 11, 2022

O-Ring Face Seal Vs. O-Ring Boss

John Joyce
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John Joyce

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The two most popular hydraulic O-ring fittings are O-ring face seal fittings and O-ring boss fittings. Yet, what’s the difference between them? While both need to create a tight seal within a hydraulic system, each serves a different purpose.


Overall, hydraulic systems involve quality seals and components to keep them operating safely and effectively.

  • One of these components, the O-ring face seal fitting (ORFS) is common in hydraulic lines on off-road construction and agricultural equipment, oil and gas, mining, and other high-performance industrial markets due to frequent actuation and vibration. An ORFS male fitting has a straight thread and O-ring in the face, where the female has a straight thread and a machined flat face. They are ideal at operating pressures of up to 6,000 psi. It’s vital to reduce or eliminate potential leak points in these and other applications, where ORFS fittings are the best line of defense. is

Another, the O-ring boss (ORB), has a male thread with an O-ring at its base that mates with a chamfer machined into the mating female connection. This type of seal makes the O-ring boss an excellent choice for medium to high pressure applications.

ORFS fittings

ORFS fittings (SAE J1453 and ISO 8434-3 standard) are made to secure an air-tight hydraulic system in high pressure applications, including harsh environments where corrosion is a threat. However, they are not suited for high temperature applications since the O-ring material can begin to break down.

The ORFS is easily reused and replaced by its intuitive design. They come in a variety of different thickness levels to fit individual tubes. An ORFS should be three to six times the cross-section diameter compared to the radii of the inner corner.

Because they have flat sealing surfaces, ORFS fittings are often paired with flanged tubing or hose fittings. Using the Buna-N or Viton, 90 durometer O-ring, the fittings are manufactured in carbon, nickel plated carbon, and stainless steel for highly corrosive applications.

ORB fittings are used in hydraulic applications that deal with medium to higher pressures. The ORB fitting comes in a variety of styles, degrees and adjustability levels, perfect for connecting two different pipes or tubes. They have become so popular that some manufacturers no longer make NPT ports on their valves or manifolds because O-ring boss ports provide a better design option than NPT. ORB fittings use threads for torque only and synthetic rubber O-rings for sealing the connection, preventing any fluid from leaking.

A lot of factors can contribute to what O-ring boss fitting to use. The measurements of the male and female internal threads will allow you to make that determination.

ORB fittings are not suited for all applications, including high temperatures, and can be improperly installed where the threaded portion of the fitting may be longer than SAE specification, resulting in the fitting bottoming out in a port.

So, what’s the difference between ORFS and ORB?

The key factor is to ensure there are no leaks within the hydraulic system by having quality O-rings and seals. While both use an encapsulated O-ring to create a seal, the difference is the location of the O-ring. ORFS has the O-ring in the grooved area at the end of the male thread, while the ORB has the O-ring at the base of the male thread.

The female ORFS engages the O-ring in the male fitting and compresses it as the fittings are threaded together, thereby creating the seal. The female ORB has a grooved “boss” before the first thread, acting to encapsulate the O-ring on the male thread.

A hydraulic system needs to be leak-free and efficient to maintain a long service life, which is largely accomplished with the selection of the correct components and an applicable system design.

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