O-Ring Cord Stock
Cut to length, any geometry, no tooling — the sealing solution when a standard molded O-ring isn't the answer.
Overview
When geometry rules out a molded O-ring
O-ring cord stock is extruded elastomer in continuous length — coiled in rolls or cut to dimension — that you splice into a closed loop at the diameter you need. It exists for cases that a catalog of molded O-rings can't cover: oversized door seals, rectangular or irregular frame seals, large-bore static seals where custom tooling would be cost-prohibitive, and field repairs where getting a molded seal shipped overnight isn't an option.
The cord carries the same chemical resistance and temperature performance as the equivalent molded compound. What it does not replicate is the dimensional precision of a molded part. Cord cross-sections hold looser tolerances — typically ±0.25 mm vs. ±0.08 mm for molded O-rings. For precision dynamic sealing in hydraulic or pneumatic cylinders, molded O-rings remain the engineering specification. For large-bore static sealing and custom geometries, cord is the right tool.
We supply cord in all standard cross-sections and all major elastomer families, in rolls or cut to your specified circumference.
Design Reference
Selection & application guidance
Cord vs molded O-ring — when to specify each
The decision is driven by three factors: bore size, geometry complexity, and service conditions. Cord is never a precision substitute for a molded seal — but for static large-bore and irregular geometry applications, it is often the only practical option.
| Criterion | Molded O-ring | O-ring Cord |
|---|---|---|
| Cross-section tolerance | ±0.05–0.10 mm (AS568/ISO 3601) | ±0.20–0.30 mm (RMA extrusion tolerance) |
| Dynamic sealing | Yes — recommended for all reciprocating, rotary, and oscillating service | No — splice joint cannot withstand cyclic loading. Static service only. |
| Large bore (>500 mm ID) | Requires custom tooling — long lead times and high minimum orders | Cut to any circumference from stock — no tooling required |
| Non-circular geometry | Not possible with standard O-ring tooling | Cord follows any groove profile: rectangular, oval, or complex frame shapes |
| Field repair | Requires correct size available | Cut from roll to any size on-site — no waiting on a specific part number |
| Pressure rating (static) | Up to 350 bar with backup rings | Practical limit depends on joint method — see our vulcanization page for details |
RMA extrusion tolerances — what to expect vs a molded O-ring
Cord stock is an extruded product, not a molded one. The die and extrusion process introduce more variation than compression moulding — both in diameter (cross-section size) and in ovality (the difference between the widest and narrowest measurement at a given cross-section). Understanding these numbers is essential before specifying cord for a tight compression window.
The table below reflects RMA extrusion tolerances for elastomeric cord stock alongside ISO 3601 molded O-ring tolerances for the same diameters, so the tradeoff is visible at a glance.
| Nominal Cord Ø (mm) | RMA Cord Tolerance (±) | RMA Max Ovality | ISO 3601 Molded O-ring (±) | AS568 Equivalent Wire |
|---|---|---|---|---|
| Up to 2.0 | ± 0.15 mm | 0.13 mm | ± 0.05–0.07 mm | 100-series (1.78 mm) |
| 2.0 – 3.0 | ± 0.20 mm | 0.15 mm | ± 0.07–0.09 mm | 100/200-series (2.62 mm) |
| 3.0 – 6.0 | ± 0.25 mm | 0.20 mm | ± 0.08–0.10 mm | 200/300-series (3.53 mm) |
| 6.0 – 10.0 | ± 0.30 mm | 0.25 mm | ± 0.10–0.13 mm | 400-series (6.99 mm) |
| 10.0 – 16.0 | ± 0.40 mm | 0.30 mm | ± 0.13–0.15 mm | Large bore custom |
| 16.0 – 25.0 | ± 0.50 mm | 0.40 mm | ± 0.15–0.20 mm | Large bore custom |
| Over 25.0 | ± 0.60 mm | 0.50 mm | Custom moulding required | — |
Ovality is the difference between the maximum and minimum cross-section diameter measured at the same point. A cord that is oval rather than round will compress unevenly in the groove — one axis is over-compressed while the other is under-compressed. This is why cord is specified for static sealing only: ovality under dynamic motion creates a leak path that reforms every stroke.
Tolerance impact on groove design & compression
Groove dimensions for cord stock use the same depth and width formulas as for molded O-rings. The difference is that you must design to the tolerance band, not the nominal diameter. A cord at its upper tolerance limit compresses more than designed; at its lower limit, it may not seal at all if the groove was cut for nominal only.
| Cord Size (nominal) | At Lower Tolerance | At Nominal | At Upper Tolerance | Design Strategy |
|---|---|---|---|---|
| 3.53 mm (300-series) | 3.28 mm → ~7% compression in standard groove | 3.53 mm → ~15% compression | 3.78 mm → ~23% compression | Target groove depth for 12–15% at nominal. At lower tolerance you still seal; at upper you're within static limits. Do not design for max compression at nominal. |
| 6.99 mm (400-series) | 6.69 mm → may fall below minimum seal contact | 6.99 mm → ~15% compression | 7.29 mm → ~23% compression | For large cross-sections, confirm minimum compression at lower tolerance. A groove designed for 20% nominal compression gives only 12% at lower tolerance — still adequate for static sealing. |
For critical static sealing above 50 bar: specify factory-vulcanized loops with a tight cross-section callout, or consider switching to a custom-tooled molded O-ring. See our vulcanization page for joint methods and pressure ratings.
Material Quick Reference
Available elastomers in cord stock
All compounds carry the same chemical and thermal resistance as their molded equivalents. Not every cross-section is available in every material — contact us to confirm what we can get for your diameter and compound combination.
NBR — Nitrile
−40 °C to +120 °C
Petroleum oils, hydraulic fluids, fuels, water. Most widely stocked cord compound. Default for industrial static sealing.
EPDM
−55 °C to +150 °C
Steam, hot water, brake fluid, phosphate-ester fluids. Common for door seals in food and pharmaceutical processing equipment.
FKM — Viton
−20 °C to +200 °C
Aggressive chemicals, fuel, high temperature. Used in reactor flanges, chemical processing, and high-temperature oven door seals.
Neoprene (CR)
−40 °C to +120 °C
Ozone, weathering, refrigerants. Common for outdoor enclosure seals, HVAC equipment, and marine applications.
Silicone (VMQ)
−60 °C to +200 °C
Extreme temperature range, food contact, oven and autoclave door seals. Low tensile strength — static service only.
TPU — Polyurethane
−40 °C to +80 °C
High abrasion resistance, good mechanical strength. Hydraulic cylinder rod wiper seals and sliding door seals where wear is the failure mode.
Need O-ring cord or factory-vulcanized loops?
Give us the cross-section diameter, inside diameter or circumference, material, and quantity. We'll confirm availability and quote fast.
Request a quote