O-Rings – From Buna O-rings to X-rings

We have one of the world’s most readily available inventories of o-rings and related accessories

Three simple words, but they have a big impact on how your business performs. We know that having the right o-ring at the right time is crucial to your operation because industrial o-rings are our business. DICHTOMATIK carries more than 10,000 SKUs including the broadest range of top quality industry standard parts, all available with material batch traceability and cure date.

And if you need something custom, our engineering and design team can help assist with your unique o-ring requirements. Our distribution and fulfillment operation is available to assist and improve your ability to deliver on orders, provide the tools to help you sell more products to your customers, and increase your bottom line by ensuring you have the right product in stock.

When you work with DICHTOMATIK, expect solutions.



From a sealing standpoint, two characteristics make an o-ring.  An o-ring—the shape and the material of construction.

The shape is a circular torus or doughnut-shaped ring (see diagram at right). As can be seen, only two of the three primary dimensions—the inner diameter (ID), the outer diameter (OD) and the cross-section (CS)—are required to completely specify the size of an o-ring.


DICHTOMATIK offers o-rings in almost any size and in a wide range of materials. We are tooled on most standard o-ring sizes and maintain a large inventory of all AS568 sizes in many different materials.  You can also download a standard size master list that includes all ID/CS combinations available in universal standard listings including SAE AS568, ISO 3601, DIN 3771, BSI BS 4518, BSI BS 1806, JIS B 2401, and NF T 47-501 sorted by CS, then by ID.


To determine the size, you will need to first measure your o-ring(s).  Measuring an O-Ring is quite simple when you have the right tools at your disposal. All that is required is a clean, level surface; an o-ring; and a measuring device such as a caliper or other measuring tools such as cones, gauges, and size charts all available from DICHTOMATIK.  For detailed information and measuring requirements, start here.


Our O-Ring offering now includes a larger selection of food grade seals that meet FDA, 3A, NSF61, and other hygienic standards which offer superior chemical resistance to cleaning agents commonly used on food & beverage production equipment in many industries including dairy, distilleries, breweries, wineries, bakeries, produce processing, meat processing, and many more.   Click on the image to the right to download our complete DICHTOMATIK O-Ring Part Number Nomenclature that include our food & beverage o-ring products as well as our vast range of other o-ring materials as your guide to finding our products on our ecatalog.


An o-ring seal has two basic components:

  • The o-ring
  • The enclosed space that both compresses and contains the o-ring

The compression (or more accurately the deformation) of the o-ring provides part of the sealing function. An additional sealing function is realized when the o-ring is activated by the pressure of the gas or liquid that the o-ring serves to contain. The containment ensures that the sealing function is maintained by keeping the o-ring where it needs to be.



Most static o-ring seals are one of the three types shown below. In the male gland seal the groove for the o-ring is machined into the piston (the part that is inserted into the bore) and that part with the o-ring installed on it is inserted into the bore. The o-ring seals radially. In the female gland seal the groove for the o-ring is machined into the bore and a smooth rod is inserted through the installed o-ring. As with the male gland seal, the o-ring seals radially. For the face seal, the groove is machined into the face that is perpendicular to the piston or rod. The o-ring seals axially.

The variable names presented in these diagrams are used throughout our design guide.  For detailed information download our O-Ring Handbook.


These tables are intended to assist the user in determining the suitability of various elastomers in many different chemical environments. The ratings are based on a combination of published literature, laboratory tests, actual field experience, and informed judgments. As laboratory tests do not necessarily predict end-use performance, users of DICHTOMATIK products should conduct their own evaluations to determine application suitability.  Download the complete DICHTOMATIK Chemical Compatability Guide and use this rating scale below to determine the best material to use in your application.



Rating Description Volume Change Comments
1 Little or no effect <10% Elastomer may exhibit slight swelling and/or loss of physical properties under severe conditions.
2 Possible loss of physical properties 10–20% Elastomer may exhibit swelling in addition to a change in physical properties.
3 Noticeable change 20–40% Elastomer exhibits a noticeable change in swelling and physical properties. Questionsable performance in most applications.
4 Excessive change >40% Elastomer not suitable for service.
Insufficient information n/a Insufficient information available for rating.


Details are provided here for some of the more common o-ring sealing elastomers. General descriptions are provided in the text. Additional details on several of these elastomers listed in this table are available for download.  This elastomer property chart will provide you with an overview of basic properties such as recommended operating temperature ranges, and resistance to common factors.  The table below outlines each abbreviation listed on elastomer properties chart along with the full description of the rubber material.


For additional information, material test reports are available for download in the material data and technical information section of our Brochures/Library webpage.

Click on the image to Download Chart.
Abbreviation Rubber Description
ACM Polyacrylate Rubber
TFE/P Aflas® (Tetrafluoroethylene/Propylene)
AU Polyurethane
CR Neoprene/Chloroprene
EPDM Ethylene-Propylene-Diene Rubber
FFKM Perfluoroelastomer
FKM Fluorocarbon Elastomer
FKM-ETP ETP Based Fluorocarbon Elastomer
FVMQ Fluorosilicone Rubber
HNBR Hydrogenated Nitrile (or HSN)
IIR Butyl Rubber
NBR Nitrile or Buna-N (Acrylonitrile-Butadiene Rubber)
NR Natural Rubber
SBR Styrene-Butadiene Rubber
VMQ Silicone Rubber


Many factors other than the gland design, specified o-ring size and elastomer selection can impact the performance of the o-ring in a sealing application. These additional factors have to do with manufacturing, inspecting and storing the o-rings properly and are typically addressed by a good quality assurance program.

Three factors that are of particular importance when dealing with o-rings are:

Manufacturing Quality Systems

The vast majority of o-rings that DICHTOMATIK offers are manufactured in facilities that are ISO-9001 certified. While these quality systems do not guarantee perfect parts, they do typically justify the associated overhead and the resulting increased cost by ensuring that the parts are manufactured, inspected and handled in a consistent manner. 

Surface Quality

The surface quality of an o-ring has a significant impact on its sealing performance. Several industry standards exist that define surface quality defect types and set maximum acceptable sizes for each defect type. For assistance our Engineers are here to help.

Storage/Shelf Life

Click on the image to Download the PDF file.


Extrusion or Nibbling

The seal develops ragged edges, generally on the low-pressure side, which appear tattered. This condition is more common with high pressure systems.

Over Compression

The seal exhibits parallel flat surfaces corresponding to the sealing surfaces. May also develop circumferential splits within the flattened surfaces.

Heat Hardening/Thermal Degradation

The seal may exhibit radial cracking on the highest temperature surfaces, often accompanied by the flattening of the seal characteristic of over-compression. Certain elastomers may exhibit signs of softening, such as a shiny surface.

Spiral Failure

The seal surface exhibits a series of deep, spiral, 45°-angle cuts. This failure is often seen with long-stroke, hydraulic piston seals.

Chemical Degradation

The seal may exhibit many signs of degradation including blisters, cracks, voids or discoloration. However, in some cases the degradation is only detectable by measurement of physical properties.

Explosive Decompression

Explosive decompression results when high-pressure gases are absorbed by the seal, and then, as the pressure is rapidly dropped, the expanding gasses are trapped in the micropores of the elastomer, causing surface blisters and ruptures as they escape.

The effected seals will exhibit random short splits or ruptures deep into the seal cross-section. When first removed the surface may also be covered with small blisters.


Abrasion occurs only with dynamic seals—seals involved with a rotary, oscillating or reciprocating motion. The seal or parts of the seal exhibit a single flat surface parallel to the direction of motion. Loose particles and scrapes may be found on the seal surface.

Plasticizer Extraction

Seen primarily in fuel systems, plasticizer extraction is characterized by a loss of volume or weight of the seal. It is often difficult to detect with only a visual inspection.

Installation Damage

The seal or parts of the seal may exhibit small cuts, nicks or gashes.

Weather Or Ozone Cracking

Occurring in seals exposed to ozone, UV radiation or other air pollutants, weather or ozone cracking is characterized by small surface cracks perpendicular to the direction of stress.