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7 Things You Should Know About O-Rings

An o-ring is a torus (doughnut) shaped ring that is used to prevent leakage of fluids and gases by filling a clearance gap between two hard surfaces. If you work with industrial machinery, chances are you’ve come across o-rings on a number of occasions. While the humble o-ring may seem unassuming, there are several things you might not know about them, and many factors worth considering to make it easier when ordering your next replacement part.

1. O-rings have remained relatively unchanged for a century

O-rings were first introduced back at the dawn of the 20th century and, despite their simplicity and the basic nature of the role they perform, the o-ring remains a straightforward and reliable solution to a problem that was first solved more than a century ago. While there has been some evolution in terms of the materials used, their hardness and sizing, they have remained otherwise unchanged in the intervening 120+ years.

2. They are used in a wide range of applications

O-rings are ideal for static applications such as pipes, cylinders, flanges and plates. Industrial applications for o-rings can include within engines, in plumbing, food production and manufacturing, Co2 seals and aircraft seals. In addition to machinery, they can be used in cameras, refrigerators, scuba gear and even the tap at your favourite pub.

3. Design failures can be catastrophic

When designing an o-ring seal, material compounds, sizing, groove design and environmental aspects are all vitally important. Get these factors incorrect and the consequences can be expensive and extremely dangerous.

A catastrophic example of a failed o-ring seal is the space shuttle Challenger disaster of 1986. In this instance a combination of temperature incompatibility and poor groove design were noted as primary causes of failure.

4. O-rings can be made of numerous materials

Rubber o-rings are possibly the most common type as they serve a wide array of purposes. O-ring rubber seals are available in an extensive range of gasket and o-ring types, including natural rubber and a wide array of other elastomer and compound products.

Considerations: Many basic rubbers and polymers aren’t suitable for use with oils, fuels or solvent-based compounds. For example, for use in an engine, these would typically need to be created specifically from compatible hybrid materials.

Nitrile o-rings are broadly seen as a general-purpose o-ring type. These are a popular type of rubber seal gasket that’s used in a wide range of applications and environments as they are reasonably resistant against water, gasoline, petroleum oils, crude oil, propane and some hydraulic fluids.

Considerations: Nitrile rubber o-rings are also a fairly robust type of seal that holds up well in physically demanding roles. However, phosphate ester hydraulic fluids, brake fluids and halogenated hydrocarbons are damaging to nitrile. Nitrile is not the best choice for high temperature or high pressure applications.

Silicone o-rings are mostly associated with the gasket/sealant industry standard today.

Considerations: silicone rubber compounds typically offer very good resistance to ozone, acids, water. They also offer a decent resistance to weathering and heat - making them great for outdoor applications. Standard silicone o-ring temperature range is generally quite wide, running anywhere from about -50 c to 200 c. However, silicone o-rings offer little shear resistance or tensile strength (although silicone seals with a higher grade of tear resistance are often offered by manufacturers).

Metal O-rings are typically created from tubing made of stainless steel.

Considerations: metal o-rings are commonly used due to their inherent resilience, in which the seals can spring back promptly once pressure is reduced. They provide robust sealing properties in static applications, however are often unsuitable for dynamic environments.

Fluorocarbon o-rings are a subgroup of elastomer seals (sometimes known as Viton).

Considerations: these offer decent resistance to high temperatures (up to 210 C), but can be prone to failure below -15 C in dynamic applications. The o-rings are often resistant to petroleum oils, acids, silicone fluids is good, as well as being highly resistant to oxidation, UV damage, fungus, ozone, oils, and petroleum-based lubricants.

NBR o-ring kits and NBR seals are found in a very large number of applications. When an o-ring compound contains the standard 32% nitrile content, it is said to be ‘medium nitrile’ (>50% it becomes a ‘high nitrile’ product, intended for use in hydrocarbon applications. <18% is considered ‘low nitrile’, suggesting for use in very low-temperature systems).

NBR 70 o-rings are often found in military, automotive and aircraft fuel systems. This is due to their oil resistance and low-temperature functionality. NBR 70 o-rings are also suitable for food-grade when compounded appropriately.

Considerations:  NBR o-ring temperature range is anywhere from -35 c to 120 c.

Buna-N o-rings are also nitrile products, however considerably cheaper than fluoroelastomers.

Considerations: these are widely used in many industries that require excellent abrasion and tear resistance, as well as suitability for use with some solvents. Potential weaknesses include limited ability to withstand particularly high temperatures, and weaker UV and weather resistance than many other o-ring types.

5. Environmental factors can affect performance

As we noted above, there are important factors to consider when selecting a suitable o-ring compound for an application. These include:

Temperature - Temperature incompatibility can greatly shorten seal life. After extended exposure to high temperatures, the o’ring will receive irreversible changes to the chemical structure of the elastomer. The o-ring will harden and become brittle, as well as potentially releasing contaminating particles into the medium.

On the other hand, changes to the elastomer from excess low temperatures are typically not permanent. Extended exposure to very low temperatures generally increases an elastomer’s hardness, but will return to a softer state as temperature rises.

Chemical compatibility – The effects of an incompatible chemical on an o’ring elastomer are generally noted via a change in volume. This might mean the o-ring swells in size and softens, or the media may extract plasticisers and other additives from the o’ring compound, causing the seal to shrink, harden and become brittle. A slight volume swell is generally fine, as it compensates for variables such as compression set. Shrinkage, however, may intensify any existing compression set problems, causing leakage and potential contamination.

Pressure – The effects of excess pressure on an o’ring seal might be an extruding into the clearance gap of the o-ring groove, or even a complete rupture of the seal itself. This may require the use of a harder compound, or a require an alteration of the o-ring groove dimensions.

Exposure to UV and radiation - prolonged exposure to UV and radiation will generally shorten the shelf life of an o-ring. Therefore, o’rings should be stored away from direct sunlight and sources of ultra violet light. Gamma radiation, for example, can have a severe effect on an elastomers compression set, allowing the elastomer to lose its compressive memory and making the seal susceptible to leaks.

6. “Standard” sizing isn’t all standard

There are many imperial and metric o’ring size standards, but few are commonly found in use.


The most common imperial standard is British Standard BS1806, which is equivalent to American Aerospace Standard AS568A.

There’s also boss seal o’rings for SAE straight thread fittings (also known as 3-series or dash-9; e.g. 3-905).

There are also non-standard sizes, some known as Dowty sizes and others based on OEM’s, however these are less common.


Compared to imperial, there are a large number of metric size standards from around the globe. For example, Japanese, British, German, Swedish, French and Italian metric standards. French and Italian standards are predominantly based on British Standard imperial, with some slight variations. The most common metric size standard in Australia is the Japanese JIS-B2401 standard, which includes pistongland and vacuum sizes (P, G & V).

Then there’s non-standard metric o-rings. Ironically, these are often more common than standards-based sizing. They are primarily based on rounded numbers, i.e 30 x 3 instead of 29.4 x 3.1, as found in JIS.

7. Grove Design Matters

Equally important as the o-ring seal itself is the groove that the o’ring seats into. It is essential that the groove be designed to accommodate not just the o’ring size, but also its intended usage. Whether it be for dynamic or static operation; radial or axial loading; vacuum or high pressure, groove design matters.

Picking the right type of o-ring for the precise task is very important if you want it to perform reliably over time. If you would like support selecting the correct o-ring for an application, please contact our team today.


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