Parker O-Rings Distributor & Custom Seals

Reference Material

What is an O-Ring?

An O-ring is a torus, or doughnut-shaped ring, generally molded from an elastomer, although O-rings are also made from PTFE and other thermoplastic materials. O-rings are used primarily for sealing.

O-rings are designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface. The seal closes off a passageway and prevents an unwanted loss or transfer of fluid. It consists of two elements, the o-ring itself and a properly designed gland to contain the elastomeric material.

O-rings are one of the most common seals used in machine design because they are inexpensive and easy to make, reliable, and have simple mounting requirements. They can seal tens of MPA (thousands of psi) pressure.

How To Measure O-Rings

Advantages of O-Rings

They seal over a wide range of pressure, temperature, and tolerance.
Ease of service, no smearing or retightening.
No critical torque on tightening, therefore unlikely to cause structural damage.
O-rings normally require very little room and are light in weight.
In many cases, an O-ring can be reused, an advantage over non-elastic flat seals and crush-type gaskets.
The duration of life in the correct application corresponds to the normal aging period of the O-ring material.
O-ring failure is normally gradual and easily identified.
Where differing amounts of compression effect the seal function (as with flat gaskets), an O-ring is not affected because metal to metal contact is generally allowed for.
They are cost-effective.

O-Ring Sealing Types

Static - application where there is little or no relative motion between the mating surfaces.

Dynamic - the o-ring functions between surfaces with definite relative motion, such as seal on the piston or hydraulic cylinder. Of the two types, dynamic sealing is more difficult and requires more critical design work and materials selection.

O-Ring Selection

O-ring selection is based on chemical compatibility, application temperature, sealing pressure, lubrication requirements, quality, quantity, and cost.

O-Ring Materials

Base polymers include:

Standard Parker O-Rings & Custom O-Rings

Industry standard o-ring sizes in accordance with AS 568 dimensions and metrics are available including Parker o-ring seals.

Custom o-rings of practically any dimension can be made to fit an application. Miniature o-rings, large special o-rings, continuously molded and spliced cord are just a few examples of the custom o-ring possibilities.

Specialty Compounds

Parker's VX065-75 Low-Temp FKM

Parker's EM163-80 Low-Temp EPDM

Frequently Asked Questions: O-Rings

How do I correctly measure an o-ring?

To measure an o-ring, focus on three critical dimensions:

Inner Diameter (ID) – the distance across the inside of the ring.
Cross-Section (CS) – the thickness of the O-Ring.
Outer Diameter (OD) – calculated by ID + 2 × CS.

ESP follows the AS568 sizing standard, which ensures consistent and interchangeable o-ring dimensions across manufacturers. Using calipers or an O-Ring measuring cone helps ensure precision.

See ESP’s blog: How To Measure O-Rings

How do I measure an o-ring if I don’t have the old one?

If the original O-Ring is missing or too damaged:

Measure the diameter of the groove where the O-Ring sits.
Measure the width and depth of the groove to estimate the cross-section.
Reference the AS568 size chart to match the correct standard O-Ring size.

See ESP’s blog: How To Measure O-Rings

What is o ring durometer hardness, and why does it matter?

O-Ring durometer is the hardness of the o-ring material. Durometer scales run numerically from 0-100, with lower numbers indicating softer materials and higher numbers for harder materials. Durometer is just shorthand for “Shore hardness”.

O-ring durometer is measured by the depth of indentation in the material created by a given force using a standardized pressure foot.

The durometer gauge has a calibrated spring which forces an indent or point into the elastomer. The correct combination of the type of sample and type of hardness gauge used are required for an acceptable hardness reading.

*Please do not try to determine durometer without a durometer gauge.

See ESP’s blog: O-Ring Durometer Scale Basics

How long do o-rings last?

The life of an o-ring depends on the compound and application environment. Extreme heat, aggressive chemicals, improper installation, or excessive pressure can shorten service life. “Shelf life” is the maximum time (beginning with manufacture date) that an o-ring or elastomeric seal – with proper packaging and storage, becomes unable to meet its original specifications.

Aerospace Recommended Practice (AP 5316) is the most comprehensive basis for establishing shelf life, however, it is not a binding specification.

Reference: ARP 5316 O-Ring Shelf Life Chart

What are the most common o-ring materials?

ESP supplies o-rings in a wide variety of engineered materials, including:

Nitrile (NBR / Buna-N): Affordable, great for oils and fuels.
FKM (Fluorocarbon / Viton®): High-temperature and chemical resistance.
FFKM (Perfluoroelastomer): Exceptional chemical and thermal resistance.
EPDM: Excellent for water, steam, and food applications.
Silicone (VMQ & FVMQ): Flexible across extreme temperature ranges.
Polyurethane (AU): High wear resistance for dynamic sealing.

Each compound is chosen based on temperature, chemical compatibility, pressure, and industry requirements (e.g., FDA, 3A Sanitary, NSF, USP Class VI).

What are best practices for o-ring installation?

Proper installation is essential for seal performance:

Inspect – verify correct size, material, and surface finish.
Clean – remove dirt, dust, and particles from the groove and O-Ring.
Lubricate – apply a compatible lubricant to reduce friction.
Install Carefully – avoid twisting, over-stretching, or pinching the O-Ring.
Inspect After Install – confirm the O-Ring is seated correctly with no visible damage.

Following these practices reduces failure risks such as extrusion, spiral failure, or compression set.

Reference: ESP O-ring Installation Guide

See ESP’s Blog: ESP's O-Ring Installation Best Practices

Material Selection Application
Material Selection