Whats in the name: O-Ring Part Numbers

What’s In The Name

          O-Ring Part Numbers      

This blog post came about from the multiple requests we have received through the years regarding what exactly these O-ring part numbers mean. We also wanted to keep things simple as we join the age of blogging. Our desire was to be informative and practical, while minimizing the potential of technical difficulties. The outcome was “What’s in the name: O-ring part numbers”. We hope you enjoy!

Lets start off by explaining what an O-ring is: An O-ring is a solid-rubber seal shaped like a doughnut or torus. When compressed between mating surfaces, an O-ring blocks the passage of liquids or gases. O-rings are available in a wide range of standard and non-standard sizes that are suitable for nearly all sealing applications. Two dimensions describe the size of an O-ring: its inside diameter (ID) and its cross-sectional diameter (CS).

The standard sizes used by major O-ring manufacturers are defined by Aerospace Standard AS568A, Aerospace Size Standard for O-rings. That document, published by the Society of Automotive Engineers (SAE), lists the sizes of O-rings in six series or groups in both inches and millimeters. The first five series are based on cross-sectional diameter: 0 series, 1 series, 2 series, 3 series, and 4 series. The sixth series, or 9 series, includes 20 sizes for boss seals. 

A typical O-Ring part number call-out might look like one of the examples below. Most are variations of the same standard make up and will tell you three things:

1. The size
2. The material and
3. The hardness (durometer).

Examples would include:

2-225 V75               2-225 N90

225-V75                  225-N90

2-225 V75D             2-225 N90D

V75-225                  N90-225

There are other variations to O-ring part numbers, but the examples seen above would be the most common. For the remainder of this post, we will be deconstructing these example part numbers to help make sense of what each character represents. 

1. The Size (-225 or 2-225): The 2- seen at the beginning is used to differentiate O-rings from other sealing products, and is a part numbering system used by some suppliers.  It may or may not be present. The three digit size number -225 is from the AS568 chart and will tell you the exact cross section and ID of the O-Ring. In this example the AS568 size is -225, which belongs to the 2 series group of O-rings, meaning the O-ring has an actual cross section of 0.139. The 25 in 225 would represent the ID, which in this case is an actual ID of 1.859. The AS658 chart will also reference these dash sizes by their “nominal” size, and for a -225 this would translate into 1-7/8 ID X 2-1/8 OD X 1/8 Cross Section. For more information regarding the AS568 chart, please go to page 21 of our O-Ring Design Guide

o ring made from nbr rubber

2. The Material or Elastomer:  The letter and the letter in the part number (V75 or N90), both represent a material code. In the case of V, the material code relates to Viton®, which is DuPont’s trade name for they’re version of an FKM material. The would associate with the material Nitrile. Below is a chart of some the most commonly used elastomer O-ring material codes and they’re meaning. 

 Material Code





Hydrogenated Nitrile Butadiene Rubber/HNBR




Carboxylated Nitrile/XNBR




Ethylene Propylene Diene Terpolymer/EPDM






3. The Hardness (75/75D or 90/90D): These numbers both represent the Durometer of the material, which is one of several measures of the hardness of a material. It will typically be a 70, 75, 80 or 90 for O-rings, although other hardness in materials are available upon special request. The Durometer scale was defined by Albert Ferdinand Shore, who developed a device to measure Shore hardness in the 1920s. The two most common scales to measure Durometer would be: Shore A and Shore D. They both use slightly different measurement systems, but both adhere to ASTM D2240 standard. The A scale is for softer materials like a rubber O-ring, while the D scale is for harder ones, such as the plastic on a hard hat. For elastomeric O-rings the scale is always Shore A. Meaning whenever you see V75, H80, or N90. All those numbers represent the Durometer (hardness) of an O-ring material on the Shore A scale. 



We hope this post was of use to anyone struggling with what exactly all these characters mean within an O-ring part number. There is a lot more to learn about O-rings and some of the information can be found in our design guide. We will be continuing on with a series of posts related to “whats in the name”, so stayed tuned for more information regarding U-Cups, T-Seals, Back-ups and more.

#o-rings sizes #o rings online #o ring material #o ring design