Square O-Ring cord is rubber cord stock with a square cross-section instead of the round cross-section used in standard molded O-Rings. It is commonly used to make custom-length static seals, spliced O-Rings, gaskets, and seals for grooves where a flat sealing surface or non-standard size is needed.
At The O-Ring Store LLC, we make getting O-Rings easy by helping customers choose the right cord size, material, and sealing approach for their application. Square O-Ring cord is especially useful when a standard molded O-Ring is not available, the seal groove is unusual, or the customer needs to make a custom seal without tooling.
What Is Square O-Ring Cord?
Square O-Ring cord is continuous elastomer cord with four flat sides. It can be cut to length and bonded into a custom ring, used as a continuous gasket, or installed into a static groove. Unlike round O-Ring cord, square cord offers a wider flat contact surface against the sealing faces.
Direct answer: Square O-Ring cord is best used for static sealing, custom gaskets, large-diameter spliced rings, non-standard grooves, and applications where a flat sealing face is preferred. It is not normally the first choice for high-speed dynamic sealing unless the gland design has been engineered for a square profile.
Common Uses for Square O-Ring Cord
- Custom spliced O-Rings: Good for large diameters, low-volume needs, repair work, and non-standard sizes.
- Static face seals: Useful between two flat surfaces where compression creates the seal.
- Gaskets for covers, lids, plates, and housings: The flat profile can provide broad contact against the mating surface.
- Replacement seals for obsolete equipment: Helpful when an original molded O-Ring size is hard to find.
- Rectangular or non-round grooves: Cord can be cut to fit custom groove shapes when the groove perimeter is known.
- Maintenance and field repair: Cord stock allows a seal to be made when a molded O-Ring is not immediately available.
Customer benefit: Square O-Ring cord gives maintenance teams more flexibility. Instead of waiting for a molded custom O-Ring, a seal can often be made from cord stock, reducing downtime and simplifying repairs.
Square O-Ring Cord Materials Available
Buna-N Nitrile, NBR, 70 Durometer Square O-Ring Cord
Buna-N, also called Nitrile or NBR, is one of the most common sealing materials for general industrial service. It is widely used with petroleum oils, hydraulic fluids, grease, and many shop and maintenance applications.
Best for: petroleum oil, hydraulic oil, general industrial sealing, water-resistant applications, shop repair, low-cost custom seals, and many maintenance uses.
Not recommended for: ozone-heavy exposure, strong acids, brake fluid, ketones, many aggressive chemicals, or high-temperature applications beyond the compound’s rating.
Why customers choose NBR square cord: It is practical, economical, easy to work with, and well suited for many everyday sealing needs.
Viton® FKM Fluorocarbon, 75 Durometer Black Square O-Ring Cord
Viton® FKM, also called Fluorocarbon, is a higher-performance elastomer used where better heat resistance, chemical resistance, and fuel resistance are required. FKM is often selected when Buna-N does not provide enough temperature or chemical resistance.
Best for: higher-temperature sealing, fuels, many oils, many solvents, chemical processing, aerospace-style sealing requirements, fuel systems, and harsh industrial environments.
Not recommended for: some amines, ketones, low-molecular-weight esters and ethers, certain hot water and steam applications, and applications requiring very low-temperature flexibility unless the exact FKM compound is approved for it.
Why customers choose Viton® FKM square cord: It provides better resistance in demanding applications where chemical exposure or heat can shorten the life of standard NBR seals.
Viton® is a registered trademark of The Chemours Company. FKM is the generic material family name.
Square O-Ring Cord Cross Sections and Tolerances
The table below lists our square O-Ring cord cross sections and tolerances. The cross section is the width of one side of the square cord.
| Square Cord Cross Section | Tolerance | Approx. Fractional Size | Typical Use Notes |
|---|---|---|---|
| 0.099" | +/- .005" | Near 3/32" | Often used as the square-cord replacement range for AS568 0.103" round cross-section sizes. |
| 0.134" | +/- .006" | Near 1/8" | Often used as the square-cord replacement range for AS568 0.139" round cross-section sizes. |
| 0.156" | +/- .007" | 5/32" | Useful for custom grooves and non-standard seals requiring a 5/32" square profile. |
| 0.203" | +/- .007" | Near 3/16" | Often used as the square-cord replacement range for AS568 0.210" round cross-section sizes. |
| 0.266" | +/- .007" | Near 1/4" | Often used as the square-cord replacement range for AS568 0.275" round cross-section sizes. |
| 0.313" | +/- .010" | 5/16" | Custom static seals, larger cover gaskets, and non-standard grooves. |
| 0.375" | +/- .010" | 3/8" | Large static gaskets, covers, plates, and custom fabricated seals. |
| 0.437" | +/- .010" | 7/16" | Large custom grooves and heavy static sealing applications. |
| 0.500" | +/- .010" | 1/2" | Large equipment gaskets and oversized static seals. |
| 0.562" | +/- .015" | 9/16" | Large custom static sealing applications. |
| 0.625" | +/- .015" | 5/8" | Large custom seals, covers, and industrial gasket applications. |
| 0.750" | +/- .015" | 3/4" | Oversized static seals and heavy equipment gasket applications. |
| 1.000" | +/- .025" | 1" | Large custom gaskets and specialty static sealing. |
| 1.250" | +/- .035" | 1-1/4" | Very large custom gaskets and specialty static sealing. |
Which Square O-Ring Cord Sizes Replace Standard AS568 O-Ring Cross Sections?
Square O-Ring cord should not be treated as a universal drop-in replacement for every round O-Ring. However, several square cord sizes are commonly selected as replacement ranges for the major AS568 round O-Ring cross-section families in static applications.
| AS568 Round O-Ring Series | Standard Round Cross Section | Closest Square Cord Cross Section | Replacement Guidance |
|---|---|---|---|
| AS568 -0XX Series | 0.070" | No direct square cord match in this list | Use a molded AS568 O-Ring or contact us for help with a custom solution. |
| AS568 -1XX Series | 0.103" | 0.099" square cord | Common square-cord replacement range for static seals using 0.103" round O-Rings. |
| AS568 -2XX Series | 0.139" | 0.134" square cord | Common square-cord replacement range for static seals using 0.139" round O-Rings. |
| AS568 -3XX Series | 0.210" | 0.203" square cord | Common square-cord replacement range for static seals using 0.210" round O-Rings. |
| AS568 -4XX Series | 0.275" | 0.266" square cord | Common square-cord replacement range for static seals using 0.275" round O-Rings. |
| AS568 -9XX Boss Series | Mixed boss seal cross sections | No universal square cord match | Boss seals are usually application-specific. Use the correct molded boss O-Ring whenever possible. |
Important: Square cord replacement is most practical for static seals. For hydraulic cylinders, rotating shafts, reciprocating rods, high-pressure dynamic sealing, and critical OEM equipment, confirm the groove design, squeeze, gland fill, pressure, temperature, and fluid compatibility before substituting square cord for a round O-Ring.
Why Are Square Cord Cross Sections Smaller Than the Round AS568 O-Ring Cross Sections?
Square cord replacement sizes are slightly smaller than the round AS568 cross sections because a square profile behaves differently in the gland than a round profile.
For example, a 0.103" round AS568 cross section is commonly paired with approximately 0.099" square cord. A 0.139" round AS568 cross section is commonly paired with approximately 0.134" square cord. The square cord is smaller in height, but it has flat sides and more contact area against the sealing surfaces.
A square profile with the same side length as a round O-Ring diameter would put more rubber volume into the groove. That can increase compression load, raise gland fill, make installation more difficult, and create a greater chance of pinching or over-compression. Using a slightly smaller square cross section helps the cord fit more realistically in glands originally designed around a round O-Ring cross section.
Simple answer: Square O-Ring cord is smaller than the round AS568 cross section because the square shape has more rubber volume and more flat contact area than a round profile of the same height. The slightly reduced size helps control squeeze, gland fill, and installation force.
How to Select the Right Square O-Ring Cord Size
1. Identify the sealing style
Start by identifying whether the seal is a static face seal, radial seal, cover gasket, custom groove gasket, or spliced O-Ring. Square cord is usually best for static sealing.
2. Measure the groove
Measure the groove width, groove depth, and groove perimeter. The square cord cross section must allow compression without overfilling the groove.
3. Choose the material
- Choose Buna-N NBR 70 durometer for general-purpose oil, hydraulic fluid, grease, and economical industrial sealing.
- Choose Viton® FKM 75 durometer for higher heat, better fuel resistance, better chemical resistance, and more demanding environments.
4. Match the cross section
If replacing a standard AS568 round O-Ring in a static seal, use the replacement guidance table above. If the groove is custom, select the cord size based on the groove dimensions rather than the AS568 number.
5. Check squeeze and gland fill
The cord must compress enough to seal, but not so much that the gland is overfilled. Too little compression can leak. Too much compression can cause difficult assembly, excessive stress, extrusion, or early seal failure.
6. Confirm chemical and temperature compatibility
The best size will still fail if the compound is not compatible with the fluid or temperature. When in doubt, compare the application fluid, temperature range, pressure, and environment before ordering.
How to Calculate Cut Length for a Square Cord O-Ring
When making a round spliced O-Ring from square cord, calculate the cut length using the centerline of the cord.
Cut Length = 3.1416 x Desired Centerline Diameter
If you know the inside diameter:
Cut Length = 3.1416 x (Inside Diameter + Cord Cross Section)
If you know the outside diameter:
Cut Length = 3.1416 x (Outside Diameter - Cord Cross Section)
For non-round grooves, calculate the groove’s centerline perimeter. The centerline is located halfway across the cord cross section. For rectangles with rounded corners, include the straight centerline lengths and the corner radius centerline arcs.
Tip: Accurate cutting and clean bonding matter. A poor splice can become the weakest point of the seal.
Square Cord vs. Round Cord
| Feature | Square O-Ring Cord | Round O-Ring Cord |
|---|---|---|
| Cross-section shape | Square with flat sides | Round |
| Best use | Static gaskets, custom grooves, flat sealing faces | General O-Ring replacement, standard glands, many static and dynamic uses |
| Contact surface | Wider flat contact area | Line-style contact before compression |
| AS568 replacement | Possible in selected static applications | Standard choice for AS568 grooves |
| Dynamic sealing | Use caution; gland must be designed for it | More common for properly designed dynamic O-Ring glands |
| Custom fabricated seals | Very useful | Very useful |
When Should You Use Square O-Ring Cord?
Use square O-Ring cord when you need a custom static seal, a flat contact surface, a non-standard gasket, or a practical repair option for a large or obsolete seal. Square cord is especially helpful when ordering a molded custom O-Ring is not practical due to lead time, tooling, or low quantity needs.
When Should You Use a Standard Round O-Ring Instead?
Use a standard round O-Ring when the groove was designed for an AS568 O-Ring, the seal is dynamic, the application is high pressure, or the equipment manufacturer specifies a molded O-Ring. A molded O-Ring is usually the best choice for critical sealing, production assemblies, and dynamic hydraulic or pneumatic applications.
Square O-Ring Cord Selection Summary
| Application Need | Recommended Starting Point |
|---|---|
| General oil and hydraulic fluid sealing | Buna-N NBR 70 durometer square cord |
| Higher heat or fuel exposure | Viton® FKM 75 durometer square cord |
| Replacing AS568 -1XX static O-Ring | 0.099" square cord |
| Replacing AS568 -2XX static O-Ring | 0.134" square cord |
| Replacing AS568 -3XX static O-Ring | 0.203" square cord |
| Replacing AS568 -4XX static O-Ring | 0.266" square cord |
| Custom large cover gasket | Select by groove width, groove depth, and compression needs |
| Dynamic rod, piston, or rotary sealing | Use caution; confirm gland design or choose a standard molded O-Ring |
Frequently Asked Questions About Square O-Ring Cord
Can square O-Ring cord replace a round O-Ring?
Yes, in some static applications square O-Ring cord can replace a round O-Ring. The groove design, compression, gland fill, pressure, fluid, and temperature must be checked first. Square cord is not a universal drop-in replacement for every round O-Ring.
Which square cord size replaces an AS568 -2XX O-Ring?
The AS568 -2XX series uses a 0.139" round cross section. The closest square cord replacement range listed here is 0.134" square cord. This is most appropriate for static sealing applications.
Which square cord size replaces an AS568 -3XX O-Ring?
The AS568 -3XX series uses a 0.210" round cross section. The closest square cord replacement range listed here is 0.203" square cord. Always check the groove before substituting.
Which square cord size replaces an AS568 -4XX O-Ring?
The AS568 -4XX series uses a 0.275" round cross section. The closest square cord replacement range listed here is 0.266" square cord. This can work well in many static applications, but the gland must not be overfilled.
Why is square O-Ring cord slightly smaller than the round O-Ring cross section?
Square cord is slightly smaller because the square shape has more rubber volume and a wider flat contact surface than a round profile of the same height. The reduced size helps manage squeeze, gland fill, and installation force.
Is Buna-N or Viton® better for square O-Ring cord?
Buna-N NBR is a strong choice for general oil, grease, and hydraulic fluid service. Viton® FKM is better for higher heat, fuel exposure, and many chemically demanding applications. The best material depends on the fluid, temperature, pressure, and environment.
Can square O-Ring cord be used for hydraulic seals?
Square O-Ring cord may be used in some static hydraulic sealing applications, but it should be used cautiously in dynamic hydraulic applications. For rods, pistons, and rotating shafts, use the seal style recommended for the gland and operating conditions.
How do I order square O-Ring cord?
Choose the material, cross section, and length needed. If you are replacing an AS568 O-Ring, provide the AS568 dash number and application details. If the groove is custom, provide the groove width, groove depth, inside diameter or perimeter, fluid, temperature, and pressure.
Need Help Choosing Square O-Ring Cord?
Choosing the right square O-Ring cord is easier when the groove, material, and application are reviewed together. The O-Ring Store LLC helps customers match cord size, compound, and sealing style so they can reduce downtime, avoid trial-and-error ordering, and get the right seal faster.
We make getting O-Rings easy. If you are replacing a standard AS568 O-Ring, building a custom gasket, or trying to identify the right square cord size, contact our team with the dimensions and application details.