Two standards, one goal
O-rings are deceptively simple, but sizing mistakes are common. The market uses two main sizing languages: AS568 dash numbers and metric sizes. The dash system is dominant in North America, while metric is common in Europe and Asia. A good size chart should translate between them without losing precision.
AS568 defines a fixed set of sizes with nominal inside diameter and cross section values. Metric sizes are usually specified directly in millimeters as ID x cross section. The challenge is that the nominal values do not always convert cleanly, which is why you should treat the chart as a matching guide rather than a perfect conversion table.
When you build a search experience, separate the user intent from the standard. Many users know only a dash number, while others only know a metric size. A flexible chart and search tool should accept both inputs and return the nearest equivalent sizes with a warning when the fit is approximate.
What AS568 dash numbers actually represent
Each AS568 dash number corresponds to a nominal ID and cross section. The dash number does not encode dimensions directly, so you need a chart to decode it. The practical tip is to group dash numbers by cross section series, then select the closest ID in that series.
When converting to metric, keep the cross section consistent first. Stretching an o-ring by more than about 3 percent can reduce sealing life. For static applications you can tolerate slightly more stretch, but for dynamic seals you should stay conservative.
AS568 vs metric quick comparison
The table below is a high level comparison. It does not list every dash number but highlights the differences you must account for when mapping between systems. Keep this in your sourcing notes so buyers understand why a metric replacement is not always one-to-one.
| Standard | Units | Typical range | Where used |
|---|---|---|---|
| AS568 | Inches | 0.049 to 0.275 in cross section | US hydraulic, pneumatic, industrial |
| ISO 3601 Metric | Millimeters | 1.00 to 10.00 mm cross section | Europe, Asia, global OEM |
| JIS B 2401 | Millimeters | 1.00 to 10.00 mm cross section | Japan and Japanese equipment |
| Mixed catalogs | Both | Depends on vendor | Aftermarket and distributors |
How to measure an o-ring correctly
Measure the inside diameter by placing the o-ring on a flat surface without stretching it. Then measure the cross section by gently closing the calipers on the ring without compressing it. If the o-ring is worn or flattened, measure multiple spots and average the results.
When a groove is available, measure the groove diameter and cross section instead of the o-ring. Groove dimensions are the true engineering intent and help you avoid errors caused by aged seals. Always record the units and keep the original measurements for traceability.
- Measure ID and cross section in the same unit system
- Avoid stretching the ring during measurement
- Use groove dimensions when possible
Material selection changes the fit
Material is more than chemical resistance. Different elastomers compress and rebound differently, which changes sealing performance. NBR is forgiving and widely used, but it can harden with heat. FKM handles heat well, but it can be less elastic at low temperatures.
Use the material as a second filter after size. When you are forced to substitute a material, review compression set data and compatibility charts. This is especially important for automotive fluids, food-grade applications, or exposure to steam.
Hardness is another factor. A harder o-ring can reduce extrusion in high pressure systems but may leak if the groove tolerances are loose. When selecting replacements, keep hardness ranges in your database so you can refine the match beyond size and material alone.
| Material | Strengths | Limitations | Typical uses |
|---|---|---|---|
| NBR | Oil resistance, cost effective | Limited high-temp | Hydraulics, general purpose |
| FKM | High-temp, fuel resistance | Higher cost, low-temp stiffness | Automotive, chemical |
| EPDM | Water and brake fluid | Not for petroleum oils | Cooling systems, washdown |
| Silicone | Wide temp range | Low tear strength | Food, medical, electronics |
Building a practical size chart
A useful chart does more than list dimensions. It adds context such as tolerance bands, groove recommendations, and material options. Start by listing the top 200 to 300 sizes you see in your market, then add the cross reference between AS568 dash numbers and the closest metric equivalents.
For each line, capture the nominal size and a recommended range. If your suppliers use slightly different nominal values, pick a canonical version and store the alternatives in a note field. That keeps your chart consistent while still tracking real-world variation.
Include a simple confidence flag for each mapping so users know which conversions are tight and which are approximate. This small addition prevents accidental misuse of the chart in critical applications.
- Group by cross section first, then sort by ID
- Store tolerance notes and supplier-specific aliases
- Track material availability for each size
Squeeze ratio and groove design basics
The squeeze ratio is the percentage compression of the o-ring cross section when installed. Too little squeeze can leak; too much squeeze accelerates wear and can cause extrusion. Typical static seals use a higher squeeze than dynamic seals, but both require a groove that matches the chosen cross section.
When cross referencing sizes, verify the groove width and depth. A dash number that is close in ID might still have a cross section that does not suit the groove. If you only change the ID, the squeeze ratio stays similar. If you change cross section, you must revisit groove design.
If you do not have the groove data, use conservative assumptions and avoid aggressive size substitutions. For critical seals, request the original equipment drawing or measure the groove directly. This is the fastest way to avoid a mismatch that a size chart cannot reveal.
Failure modes you can read from the old ring
A flat, shiny ring usually indicates over-compression or heat, while a cracked ring suggests chemical attack or low-temperature brittleness. If the ring is extruded or nicked, the gap clearance or pressure may be too high for the material. These clues are useful when deciding whether to keep the original size or change material.
Use the failed ring as a data point, not just a replacement trigger. Photograph it, record the failure pattern, and link it to the part number in your database. Over time, this builds an internal knowledge base that improves your cross reference accuracy far more than generic charts.
Also record service intervals. If a ring consistently fails earlier than expected, the problem may be installation quality or system pressure spikes rather than the size itself. A good database links the part number to operating context so you can tell the difference between a sizing issue and a process issue.
If you see surface glazing or excessive hardening, consider whether the operating temperature is higher than expected. That might require a material upgrade rather than a size change.
When metric should not be replaced by AS568
Some equipment uses metric groove designs with tight tolerances. In those cases, the closest AS568 size may still be out of tolerance. If you see repeated leakage on cross referenced parts, inspect the groove and verify the squeeze ratio rather than forcing a conversion.
When in doubt, source the original standard. The cost of a custom or special order is often lower than the cost of repeated maintenance or equipment downtime.
If a conversion is unavoidable, document the decision and monitor early field performance. A short validation period can reveal whether the fit is acceptable before you roll out the change across a fleet of machines.