5 Things You Should Know about O-ring Face Seal Fittings

Guest brand Brennan is back for another informative post about O-ring Face Seal Fittings (ORFS). They are a highly popular and reliable choice in hydraulic and tube connections. Well designed ORFS connections provide a seal that is highly reliable due to its elastomeric seal and is one of the best choices for a leak-free connection. These elastomeric seals provide a leak-proof, drop-in replacement for SAE 37° flared tube fittings, literally eliminating hydraulic leaks when used properly. When the connection is tightened, the O-ring compresses against a flat face sleeve, which has been brazed onto the end of a tube. This makes a metal-to-metal seal with a leak barrier provided by the O-ring. The flat face sleeve also eliminates twisting of the tube and provides support during assembly.

O-ring face seal fittings can eliminate leaks in hydraulic systems at operating pressures as high as 6,000 PSI. They are designed specifically for applications where elastomeric seals are acceptable to overcome leakage and variations in assembly procedures. The fittings can be easily connected to a range of tubing grades by silver brazing. Also, ORFS fittings can be disassembled and reassembled in the field to facilitate the replacement of the O-ring or the entire assembly.

To ensure leak-free connections, here are 5 things you should know about O-ring face seal fittings:

1 – ORFS Fittings Help Prevent Component Failure

Many severe conditions can contribute to hydraulic component failure. Fittings and component assemblies are frequently required to work in very harsh environments, from corrosive off-shore applications to extreme heat and cold or frequent high-pressure fluctuations. These are a few applications where ORFS fittings can provide leak-tight connections to prevent failures.

O-ring face seal fittings are used in the majority of applications where there are very high pressures that are subject to flexing or pressure surges in construction equipment, agricultural equipment, oil and gas, mining, and the high-performance industrial markets. They are also the preferred connection in high vibration systems because the soft O-ring seal absorbs shock better than any metal-to-metal sealed fittings.

2 – Causes of Hydraulic Systems Leakage

While hydraulic connections that incorporate an O-ring face seal are the most reliable against leakage, it can still happen. Yet the cause is rarely the fitting itself. The primary contributors to system leaks and how often they occur include:

• Improper installation: 60%

• Poor system design: 20%

• Quality of components: 15%

• System abuse: 5%

ORFS fittings are the best line of defense against costly fluid leaks.

3 – Applications

O-ring face seal SAE J1453 fittings are popular in hydraulic lines onboard off-road vehicles such as tractors, combines, and other heavy equipment used in construction. Due to the frequent vibration and actuation of components in off-road vehicles, it is extremely important to reduce or eliminate the number of potential leak points and incidences of leakage. The SAE J1453 standard for ORFS fittings covers dimensions, threads, fitting and O-ring materials, and performance requirements for use in hydraulic applications, as well as the interface of the O-ring face and nut portion of hose stem assembly.

Flow meters that incorporate NPT end-fittings require adapters and can experience several leak points. However, units utilizing O-ring face seal fittings do not require adapters, need less space for meter installation, and ensure fewer possible leak points.

Another important factor in many applications that use ORFS fittings versus traditional metal-seal fittings is that manufacturers can extend warranty periods in equipment because of the reduction of possible leak points.

4 – System Design – Where it all starts

System design is the first consideration for leak prevention in hydraulic systems. The right components must be selected to reduce the incidences of leaks and connection failures. Typically O-ring face seal fittings are the best or sometimes the only practical choice. The following are attributes that should be taken into consideration when selecting system connections in the design phase.


The environment will determine the fitting design and selection. The use of stainless steel or a  protective coating on steel fittings extends the service life in corrosive environments. The most common finish for steel fittings is electroplated zinc to avoid rusting from the presence of oxygen, acidic gases, and moisture. However, stainless steel is the best choice for highly corrosive environments such as off-shore or applications exposed to caustic chemicals. Finally, the choice of O-ring material must meet the system conditions and application as well. A fluorocarbon or other material may need to be specified over Buna-N in certain applications, particularly in very high temperatures (see Temperature for Fittings & O-ring Seals chart below).


The dynamic pressure rating of the fitting assembly is to be equal to or higher than the system pressure. Typically, fittings are rated at a 4:1 design ratio, which applies to normal operating conditions that can withstand moderate hydraulic and mechanical shock. For severe vibration and shock operating conditions a “de-rating factor” should be applied directly to the dynamic pressure of the fitting as shown in the table below.

5 – O-Ring Face Seal Fittings Assembly Instructions

Proper assembly of the fittings is critical in all situations, but often in the field, short cuts are taken that can be a detriment to system performance. It is important to take the following steps:

First: Inspect the fitting for possible contamination or damage that may have occurred in shipping or storage. Confirm the O-ring is properly installed into the groove in the fitting’s face.

Second: Using the system’s hydraulic fluid or a light lubricant, lubricate the threads and the O-ring.

Third: Have the face seal flange mated against the O-ring and finger tighten the flange nut.

Fourth: Use the properly rated torque value to tighten the flange nut.

The above procedure is known as “wet torquing”, which uses the system’s hydraulic fluid to lubricate the O-ring and fitting threads before installation. It is important to note that the coefficient of friction can vary significantly between assemblies because of differences in plating, material (stainless steel versus carbon steel), and thickness, as well as the thread quality between different components. There is less torque required on the threads by lubricating before installation which produces a more consistent coefficient of friction, ultimately increasing clamping force on the sealing area. Over tightening the fitting will make the threads deform and yield which will cause them to lose adequate clamping force or load on the seating area. Severe conditions and overextending operation of the system can cause additional yielding, resulting in unwanted leak points. If the fitting is not wet torqued it will compromise the efficiency and service life of the system. The table below shows the recommended torque values per tube and fitting size.

These tips that Brennan has provided allow for the proper installation of O-ring face seals and therefore maximize the life of your machine. Be sure to check out MISUMI’s line of Brennan components here.

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  1. Alice Carroll

    January 17, 2021 at 9:50 pm

    It’s interesting to know that there are certain ways to effectively lubricate hydraulic seals. I’ve been thinking about shopping for some spare seals for a used tractor I’m planning to buy. I’m sure that I’m going to have to learn how to repair and tune it up once I start using it for my hobby farm someday.


  2. Horiaki India

    April 7, 2021 at 12:30 am

    I really like the way that you have explained many things in this blog related to O Ring Seals fittings. It will definitely help me a lot. Horiaki India is also dealing with rubber to metal bonded products.


  3. OKmarts Online Store

    November 2, 2021 at 1:23 am

    Good article, thank you for your share.


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