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X-ring vs O-ring: when a four-lobed seal makes sense
An X-ring is an O-ring with a four-lobed cross-section instead of a round one. It fits the same AS568 dash sizes and often the same groove, but it seals on two lines per side, needs less squeeze, and its lobed shape resists the rolling that produces spiral failure in reciprocating service. An O-ring stays the default for static sealing and for the widest choice of compounds and sizes.
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The two shapes side by side (AS568 sizing)
| Attribute | O-ring | X-ring (four-lobed) |
|---|---|---|
| Cross-section | Round | Four lobes with a valley between each pair |
| Sealing lines | One per side | Two per side |
| Sizing | AS568 dash number, ID and cross-section | The same AS568 series; the maker's standard tooling is sized to AS568 |
| Groove | The AS568 gland for the series | A narrower groove than the O-ring's in the maker's groove tables; a swap into an O-ring groove is checked for width and fill |
| Squeeze needed to seal | The standard squeeze for the series | Less, because two lobes share the load |
| Friction in motion | Higher | Lower, because less squeeze is needed to seal |
| Spiral twist | Prone in long-stroke reciprocating service | Resists it; the lobes stop the section rolling |
| Mold parting line | On the sealing diameter | In the valley, off the sealing surface |
| Compound range | Every elastomer family | Nitrile and FKM as the maker's standard line; other families molded to order |
| Size range | Every size on the AS568 chart, plus metric | The AS568 series in standard compounds; unusual sizes to order |
Where each ring wins
| Service | First choice | Why |
|---|---|---|
| Static face or radial seal | O-ring | No motion, so the X-ring's friction and twist advantages do not apply; wider compound and size choice |
| Reciprocating rod or piston, long stroke | X-ring | Resists spiral twist, and seals at lower friction |
| Slow rotary shaft | X-ring | Lower friction and less heat at the shaft; the section does not roll |
| Low-friction pneumatic cylinder | X-ring | Seals at less squeeze, so breakaway friction is lower |
| High pressure | O-ring with backup rings | Extrusion is governed by the gap and the hardness, and the backup ring solves it for either section |
| Retrofit into an existing O-ring groove | X-ring, same dash number | Often fits; check the groove width and the fill against the maker's groove table first |
| Metric or non-AS568 sizes | O-ring | The metric and special-size range is far wider for round sections |
| Vacuum or gas at rest | O-ring | Static service; the single sealing line is enough and the compound choice is wider |
Why four lobes change the behavior
Two sealing lines per side let the ring seal at less squeeze than a round section needs, and the maker ties that lower squeeze to lower friction and longer seal life. And because a lobed section cannot roll in the groove, the twisting that cuts an O-ring in long-stroke hydraulics does not start. The squeeze guide gives the round-section figures the lobed section improves on.
The mold parting line is the quiet advantage. On an O-ring it runs around the sealing diameter, so a flash line or a mismatch sits on the surface that has to seal. On an X-ring it sits in the valley between lobes, away from both sealing lines. The groove design guide shows where each surface of the gland meets the ring.
Sizing an X-ring from an O-ring
The dash number carries over. An X-ring is ordered by the same AS568 dash as the O-ring it replaces, and its ID and cross-section are measured the same way, with the cross-section read across the lobes. The measuring post covers the tools, and the dash number guide and the AS568 chart give the size by series.
The groove is the check. The maker's own groove tables call for a narrower groove than an O-ring's, so the gland fill and the groove width are confirmed before a swap into an existing O-ring groove is drawn. A swap on the print is the drawing owner's change; the quote follows the callout.
Compounds and callouts
Nitrile and FKM are the maker's standard X-ring compounds, other families are molded to order, and the compound callout on the print governs the part as it does for an O-ring. A brand name on the print, such as Quad-Ring®, constrains the source; without a brand or compound callout, generic X-rings in the same cross-sections meet the print. Conquest supplies X-rings and O-rings in the AS568 series and quotes to the compound and brand the print names.
Where the reason for the swap is a failed O-ring, read the ring first. The failure modes post separates the spiral failure an X-ring solves from the extrusion a backup ring solves, and the X-ring page lists what is supplied.
Quad-Ring is a trademark of Trelleborg Sealing Solutions US, Inc.; Parker® is a trademark of Parker Hannifin Corporation; no affiliation or endorsement is implied.
Sources
- Parker O-Ring Handbook ORD 5700, sections 3.5 to 3.7 (stretch, squeeze, gland fill), read 2026-09-24; section 10.1 (spiral failure), read 2026-09-27
- Minnesota Rubber and Plastics (Trelleborg), Quad-Ring brand seals design guide pages (lobed section, sealing surfaces, parting line, groove, standard compounds), read 2026-09-27
- SAE AS568 (title only)
Frequently asked questions
Can an X-ring replace an O-ring in the same groove?
Often, by the same dash number, but the maker's groove tables call for a narrower groove than an O-ring's. Confirm the groove width and the gland fill against the X-ring section before the print is changed.
Does an X-ring seal better than an O-ring?
In moving service it seals with less friction and resists spiral twist, and its two sealing surfaces give a second line of contact. In static service the O-ring offers more compounds and sizes.
Are X-rings available in every AS568 size?
The maker's standard range covers the AS568 series in nitrile and FKM. Other compounds and unusual sizes are molded to order, so the O-ring range is wider.
Why does an O-ring twist and an X-ring not?
A round section can roll in the groove as the rod slides, and the roll becomes a twist that cuts the ring. The lobes of an X-ring sit against the groove walls and stop the section rolling.
Does an X-ring need a backup ring?
At the same pressure and clearance as an O-ring, yes. Extrusion depends on the gap and the hardness, not on the section shape.
Is a Quad-Ring seal the same as an X-ring?
Quad-Ring is the original maker's trademark for the four-lobed section; X-ring is the generic name. Without a brand callout on the print the two cross in the same cross-sections.
Related guides
Orders are handled under Conquest Seal's AS9100D quality management system, whose certificate scope includes distribution.
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