O-rings
Metric O-rings
A metric O-ring is specified by its inside diameter and its cross-section in millimeters, for example 25 x 3.55, and the size lists behind that callout are ISO 3601-1 and the national systems it descended from or sits beside: DIN 3771, JIS B 2401, BS 1806 and BS 4518. Conquest Seal supplies metric O-rings in nitrile, FKM, EPDM, silicone, HNBR, FFKM and the other compound families, and matches an AS568 size to a metric callout when one exists.
- Callout
- ID x CS mmfor example 25 x 3.55
- ISO 3601-1 metric list
- 360 sizesfive cords, 1.8 to 7.0 mm
- ISO size code
- C0250group letter + ID in tenths of a mm
- Nearest AS568
- 254 of 346sizes within 2% on ID
What metric means on an O-ring print
A metric callout gives the nominal inside diameter and the cross-section in millimeters, ID x CS, and the two numbers together are the size. ISO 3601-1 adds a size code for each entry in its metric list: a cross-section group letter A to E followed by the inside diameter in tenths of a millimeter, so C0250 is 25.0 mm ID on the 3.55 mm cord. Send whichever form the print uses and the other is derived.
One trap is worth naming. Several charts titled ISO 3601 metric sizes are in fact the AS568 inch series converted to millimeters, because ISO 3601-1 also carries the inch-derived sizes and gives them the AS568 dash number as their size code. A true metric size sits on one of the five metric cords, 1.8, 2.65, 3.55, 5.3 or 7.0 mm; an AS568 size converted to millimeters sits on 1.78, 2.62, 3.53, 5.33 or 6.99 mm.
ISO 3601-1 also carries two tolerance classes and, in part 3, three quality grades. Class A tolerances are equivalent to AS568 and class B is the wider class used for the economical true-metric sizes; grade N is standard, S is increased requirements and CS is for critical applications such as aerospace and oxygen service. A print that calls out the class or grade should be quoted with it.
The metric size systems: ISO 3601-1, DIN 3771, JIS B 2401, BS 1806 and others
Each system designates a size differently, and the number on an old drawing tells you which list it came from. The standards belong to their publishers and none of their tables is reproduced here; what follows is how each callout is built and where it stands today.
| System | Issuing body, region | How a size is designated | Status and notes |
|---|---|---|---|
| ISO 3601-1 | ISO, international (also BS ISO 3601-1:2012+A1:2019) | cross-section group letter A to E plus the ID in tenths of a mm (C0250 = 25.0 x 3.55); the inch-derived sizes keep the AS568 dash as their code | current; 360 metric sizes on the 1.8, 2.65, 3.55, 5.3 and 7.0 mm cords, plus the AS568-derived inch list |
| DIN 3771 | DIN, Germany | nominal ID x CS in mm (40 x 3.55), optionally a quality letter and a polymer and hardness code | withdrawn; DIN 3771-1:1984-12 was replaced by DIN ISO 3601-1:2010-08. Its metric list is the ancestor of the ISO 3601-1 list, so an old German callout usually lands on an ISO size exactly |
| JIS B 2401 | JSA, Japan | series letter plus an index number close to, but not equal to, the ID in mm (P22 = 21.8 x 2.4); P moving, G fixed, S compact, V vacuum flange | current (JIS B 2401-1:2012); 258 sizes; P cords 1.9, 2.4, 3.5, 5.7 and 8.4 mm, G 3.1 and 5.7, S 1.5 and 2.0, V 4.0, 6.0 and 10.0 |
| BS 1806 | BSI, United Kingdom | dash number; for every size AS568 also lists, the same dash number and the same dimensions | superseded by BS ISO 3601, still quoted on drawings; adds between-sizes in 5xx, 6xx and 8xx blocks that AS568 does not have |
| BS 4518 | BSI, United Kingdom | four digits = ID in tenths of a mm, hyphen, two digits = CS in tenths of a mm | legacy metric British series on 1.6, 2.4, 3.0, 5.7 and 8.4 mm cords; none of them is an ISO 3601 cord |
| SMS 1586 | SIS, Sweden | ID x CS as DIN 3771; rings grouped D (dynamic or static) or S (static) | legacy; the same 1.6, 2.4, 3.0, 5.7 and 8.4 mm cord family as BS 4518 |
| NF T 47-501 | AFNOR, France | group letter A to E, four ID digits, a precision-class letter (A aerospace, G general) and a visual-class letter (N or S) | legacy; very close to ISO 3601-1 in both sizes and code |
| SAE AS568 | SAE, United States | three-digit dash number, a catalog lookup (-214 = 0.984 in ID x 0.139 in) | current; the inch system, 349 general-purpose sizes plus 20 boss sizes |
Common metric cross-sections and the AS568 cord they sit beside
The five ISO 3601-1 metric cords each sit within tolerance of an AS568 cord, so the cords interchange and the question is only the inside diameter. The JIS, BS 4518 and SMS cords are separate families: a 2.4 or 5.7 mm ring is not an ISO or AS568 cord, and a 2.0 or 1.5 mm ring is a JIS S-series size with no counterpart in either.
| Cross-section, mm | System and series | AS568 cord it sits beside, mm | Cords interchange? |
|---|---|---|---|
| 1.5 | JIS B 2401 S series | none | no counterpart |
| 1.6 | BS 4518, SMS 1586 | none | separate family |
| 1.78 | AS568 0xx series (0.070 in) | 1.78 | the AS568 cord |
| 1.8 | ISO 3601-1 group A | 1.78 | yes, within tolerance |
| 1.9 | JIS B 2401 P series | 1.78 | marginal overlap |
| 2.0 | JIS B 2401 S series | none | no counterpart |
| 2.4 | JIS B 2401 P series, BS 4518, SMS 1586 | 2.62 | no |
| 2.62 | AS568 1xx series (0.103 in) | 2.62 | the AS568 cord |
| 2.65 | ISO 3601-1 group B | 2.62 | yes, within tolerance |
| 3.0 | BS 4518, SMS 1586 | none | separate family |
| 3.1 | JIS B 2401 G series | none | no counterpart |
| 3.5 | JIS B 2401 P series | 3.53 | yes, within tolerance |
| 3.53 | AS568 2xx series (0.139 in) | 3.53 | the AS568 cord |
| 3.55 | ISO 3601-1 group C | 3.53 | yes, within tolerance |
| 4.0 | JIS B 2401 V series | none | no counterpart |
| 5.3 | ISO 3601-1 group D | 5.33 | yes, within tolerance |
| 5.33 | AS568 3xx series (0.210 in) | 5.33 | the AS568 cord |
| 5.7 | JIS B 2401 P and G series, BS 4518, SMS 1586 | 5.33 | no |
| 6.0 | JIS B 2401 V series | 6.99 | no |
| 6.99 | AS568 4xx series (0.275 in) | 6.99 | the AS568 cord |
| 7.0 | ISO 3601-1 group E | 6.99 | yes, within tolerance |
| 8.4 | JIS B 2401 P series, BS 4518, SMS 1586 | none | separate family |
| 10.0 | JIS B 2401 V series | none | no counterpart |
Nearest AS568 size for a metric callout: worked examples
Because the cords interchange, an AS568 size can stand in for an ISO 3601-1 metric size when the inside diameters are close enough. The rule used here comes from published stretch guidance: within 2 percent on inside diameter the two sizes are dimensionally close and the gland still has to be checked; 2 to 5 percent means the cord flattens and the groove must be recomputed; more than 5 percent is not a substitute. The examples are our own compilation from the two size lists, not a table from either standard.
| AS568 dash | AS568 ID x CS, mm | Nearest ISO 3601-1 | ISO ID x CS, mm | ID difference, % | Verdict |
|---|---|---|---|---|---|
| -006 | 2.90 x 1.78 | A0028 | 2.8 x 1.8 | -3.45 | check gland |
| -010 | 6.07 x 1.78 | A0060 | 6.0 x 1.8 | -1.15 | within 2 percent on inside diameter |
| -112 | 12.37 x 2.62 | B0125 | 12.5 x 2.65 | +1.05 | within 2 percent on inside diameter |
| -114 | 15.54 x 2.62 | B0160 | 16.0 x 2.65 | +2.96 | check gland |
| -206 | 12.29 x 3.53 | C0140 | 14.0 x 3.55 | +13.91 | not a substitute |
| -214 | 25.00 x 3.53 | C0250 | 25.0 x 3.55 | 0.00 | within 2 percent on inside diameter |
| -218 | 31.34 x 3.53 | C0315 | 31.5 x 3.55 | +0.51 | within 2 percent on inside diameter |
| -425 | 113.67 x 6.99 | E1150 | 115.0 x 7.0 | +1.17 | within 2 percent on inside diameter |
Across the 346 AS568 general-purpose sizes on the five standard cords (-001 to -003 sit below them), 254 have an ISO 3601-1 metric size within 2 percent, 22 fall in the check-gland band, 20 are not substitutes and 50 sit outside the reach of their cross-section group. Each AS568 size page carries its own dimensions; the metric size guide walks through reading and converting a callout.
How to specify a metric O-ring
Send the inside diameter and the cross-section in millimeters, or the ISO 3601-1 size code, exactly as the print shows them. Add the compound and hardness, the quantity, and the tolerance class or quality grade if the print calls one out. If the callout is an old DIN, JIS or BS number, send it as printed with the drawing and Conquest Seal reads it against the size lists on file before quoting.
A metric size that has a close AS568 counterpart is quoted both ways when the two sizes are within 2 percent on inside diameter, so you can see both options side by side. The print governs; any change of size or standard is the buyer's engineering decision. A size that only exists on a JIS or BS 4518 cord is quoted as that size.
| Family | ASTM D1418 | Standard range, C | Typical media |
|---|---|---|---|
| Viton® | FKM | -26 to 204 | fuels, oils, heat |
| Buna-N | NBR | -34 to 100 | mineral oils, hydraulic fluid |
| EPDM | EPDM | -57 to 150 | water, steam, glycol; never petroleum oils |
| Silicone | VMQ | -54 to 204 | dry heat and cold, static service |
| FFKM | FFKM | -26 to 320 | aggressive chemistry and temperature beyond FKM |
| HNBR | HNBR | -32 to 149 | oil with heat |
| Fluorosilicone | FVMQ | -73 to 177 | fuels at low temperature |
| Polyurethane | AU/EU | -40 to 82 | abrasion and high pressure |
| Aflas® | FEPM | -9 to 232 | steam, amines and sour gas; poor at low temperature |
| Butyl | IIR | -59 to 121 | gas impermeability, brake fluid and phosphate esters; never oil |
| Polyacrylate | ACM | -21 to 177 | hot transmission oil; poor at low temperature and in water |
| Ethylene-acrylic | AEM | -40 to 149 | hot automotive oil |
| SBR | SBR | no range on file | the compound sheet governs |
| Natural rubber | NR | no range on file | the compound sheet governs |
| Epichlorohydrin | ECO | no range on file | the compound sheet governs |
Related pages
Larger and non-standard sizes: a metric O-ring beyond the ISO 3601-1 list, or on a diameter no list reaches, is supplied to order as a spliced and vulcanized O-ring in any compound family made as cord.
Frequently asked
What is a metric O-ring and how do I specify one?
A metric O-ring is sized in millimeters, by inside diameter and cross-section, on one of the metric size lists: ISO 3601-1 today, DIN 3771 and JIS B 2401 on older or Japanese drawings. Specify it as ID x CS in mm or as the ISO size code, add the compound, hardness and quantity, and quote the tolerance class or quality grade if the print names one.
Is a 25 x 3.55 O-ring the same as an AS568-214?
They are the nearest thing to the same size in the two systems: ISO 3601-1 C0250 is 25.0 mm ID on the 3.55 mm cord and AS568-214 is 25.00 mm ID on the 3.53 mm cord, so the inside diameters match and the cords interchange within tolerance. The tolerance class on the print decides whether the substitution is acceptable; class A tolerances are the AS568 ones.
What is a 2 mm O-ring?
A 2.0 mm cross-section is a JIS B 2401 S-series cord, and it has no counterpart in ISO 3601-1 or AS568; the nearest ISO cord is 1.8 mm and the nearest AS568 cord is 1.78 mm. If a print says 2 mm, confirm whether it means the JIS cord or a rounded 1.8 mm before ordering.
What does an O-ring 20 mm callout mean?
20 mm on its own is an inside diameter, not a size. Add the cross-section: 20 x 2.65 is the ISO 3601-1 size B0200, for example, and the same inside diameter can exist on other cords. The print or the groove gives the cross-section.
Why does my ISO 3601 chart show inch sizes?
Because ISO 3601-1 carries both lists: a true metric list on the 1.8, 2.65, 3.55, 5.3 and 7.0 mm cords and the AS568-derived inch sizes, which keep the AS568 dash number as their ISO size code. A chart that only shows 1.78, 2.62, 3.53, 5.33 and 6.99 mm cords is the inch series in millimeters.
Do metric O-rings come in the same compounds?
Yes. Nitrile, FKM, EPDM, silicone, HNBR, fluorosilicone, polyurethane and FFKM are supplied in metric sizes the same way as in AS568 sizes; the media and the temperature pick the family and the compound sheet sets the limit.
Related guides
Sources
Size systems, designation rules, cross-section families and the ISO 3601-1 class and grade letters; Nearest-AS568 worked examples and the verdict counts; AS568 dimensions; Parker O-Ring Handbook ORD 5700 compound family ranges. Dimensional facts are stated in our own words; no standard's text or table is reproduced.
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Send the metric callout as printed
ID x CS in millimeters or the ISO 3601-1 size code, the compound and the quantity. A Conquest representative replies with the quote, and with the AS568 counterpart when one is within 2 percent on inside diameter.
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