Reference guide
AS568 Dash Numbers Explained
An AS568 dash number is a three-digit size code that stands for one nominal inside diameter and one cross-section in the SAE AS568 table, so it is looked up rather than calculated. The first digit names the cross-section family, from 0 for 1/16 in to 4 for 1/4 in, plus 9 for the tube-fitting boss seals, and a higher number within a family means a larger inside diameter: AS568-214 is a 0.139 in cross-section with a 0.984 in inside diameter. The current revision, AS568F, lists 369 sizes.
An AS568 dash number is a catalog lookup, not a formula: each three-digit number maps to exactly one nominal inside diameter and one cross-section. The first digit tells you the cross-section family (0 for 1/16 in, 1 for 3/32 in, 2 for 1/8 in, 3 for 3/16 in, 4 for 1/4 in, 9 for the tube-fitting boss series), and within a family a higher number means a larger inside diameter. AS568-214, for example, is a 1/8 in cross-section ring (0.139 in, 3.53 mm) with a 0.984 in (24.99 mm) inside diameter. The current revision, SAE AS568F, defines 349 general-purpose sizes plus 20 boss seals, 369 in all, from a 0.029 in inside diameter at dash -001 to 25.94 in at the top of the 3/16 in and 1/4 in families. This guide decodes the number, the prefixes other makers put in front of it, and the mil-spec part numbers that carry it.
The dash-number families
Counts and ranges below are taken from the AS568 size tables in the Parker® O-Ring Handbook ORD 5700 (Table 9-1, Parker Series 2-XXX O-Ring Sizes, pages 9-2 to 9-10, and Table 9-2, Parker Series 3-XXX O-Ring Sizes, page 9-11, read 2026-10-13); the tolerances widen with diameter, which is why the range is given rather than one figure.
| Dash range | Sizes | Nominal CS | Actual CS (in / mm) | Inside diameters (in) | ID tolerance (in) |
|---|---|---|---|---|---|
| -001 to -050 | 50 | 1/16 in | 0.070 / 1.78 (below -004: 0.040, 0.050, 0.060) | 0.029 to 5.239 | 0.004 to 0.037 |
| -102 to -178 | 77 | 3/32 in | 0.103 / 2.62 | 0.049 to 9.737 | 0.005 to 0.055 |
| -201 to -284 | 84 | 1/8 in | 0.139 / 3.53 | 0.171 to 17.955 | 0.005 to 0.085 |
| -309 to -395 | 87 | 3/16 in | 0.210 / 5.33 | 0.412 to 25.940 | 0.005 to 0.120 |
| -425 to -475 | 51 | 1/4 in | 0.275 / 6.99 | 4.475 to 25.940 | 0.033 to 0.120 |
| -901 to -932 | 20 | tube-fitting boss series | 0.056 to 0.118 by size / 1.42 to 3.00 | 0.185 to 2.337 | 0.005 to 0.018 |
Outside diameter is never in the callout; it is the inside diameter plus twice the cross-section. Note the gaps in the numbering: there is no -051 to -101, no -179 to -200, no -285 to -308 and no -396 to -424. A dash number in one of those gaps is not an AS568 size. The 1/16 in family begins at -004; the three sizes below it (-001, -002, -003) sit on smaller cords of their own. The full table with tolerances is the AS568 size chart.
Worked examples: reading five callouts
| Dash | Family | ID (in / mm) | CS (in / mm) | ID tolerance (in / mm) | Metric neighbor |
|---|---|---|---|---|---|
| -001 | smallest, below 1/16 in | 0.029 / 0.74 | 0.040 / 1.02 | 0.004 / 0.10 | none; no ISO metric cord this small |
| -010 | 1/16 in | 0.239 / 6.07 | 0.070 / 1.78 | 0.005 / 0.13 | ISO A0060, 6.0 x 1.8 mm, within 2 percent |
| -112 | 3/32 in | 0.487 / 12.37 | 0.103 / 2.62 | 0.005 / 0.13 | ISO B0125, 12.5 x 2.65 mm, within 2 percent |
| -214 | 1/8 in | 0.984 / 24.99 | 0.139 / 3.53 | 0.010 / 0.25 | ISO C0250, 25.0 x 3.55 mm, within 2 percent |
| -325 | 3/16 in | 1.475 / 37.47 | 0.210 / 5.33 | 0.015 / 0.38 | ISO D0375, 37.5 x 5.3 mm, within 2 percent |
| -425 | 1/4 in | 4.475 / 113.67 | 0.275 / 6.99 | 0.033 / 0.84 | ISO E1150, 115 x 7 mm, within 2 percent |
| -901 | boss series | 0.185 / 4.70 | 0.056 / 1.42 | 0.005 / 0.13 | none; metric fittings use ISO 6149 |
The ISO metric sizes come from the Precision Polymer Engineering ISO 3601 O-ring Sizes reference table, read 2026-09-06 and 2026-09-28. The metric O-ring sizes guide explains the systems.
What do the Parker 2-XXX and other prefixes mean?
Parker's 2-XXX numbers are the AS568 sizes with a prefix (2-214 is AS568-214); 3-XXX are boss gaskets, 5-XXX are Parker specials outside the standard, and 8-XXX are military cross-references. A full Parker callout adds the compound, for example 2-214 N0674-70: N for nitrile, 0674 the compound number, 70 the Shore A durometer. Other makers use the same dash number with their own compound codes, so the dash size is the part of the callout that travels between suppliers. The 2-XXX to AS568 cross-reference chart lists the sizes with the metric conversion.
Which mil-spec part numbers carry the same dash size?
Military O-ring part numbers are the AS568 dash number with a material specification in front of it. The size is identical; the prefix tells you the compound and the fluid it was qualified for. The pairings below are from the Parker O-Ring Handbook ORD 5700, Tables 8-3 and 8-4 (pages 8-4 and 8-5, read 2026-09-24), with the document statuses from the DLA ASSIST document records listed under Sources.
| Part number form | Material specification | Compound | Qualified fluid or service | Size system |
|---|---|---|---|---|
| MS28775-XXX | MIL-P-25732, now MIL-PRF-25732 (inactive in DLA ASSIST, last revised 2024) | Nitrile, 75 Shore A (Parker N0304-75) | MIL-H-5606 hydraulic fluid | AS568 dash |
| M83248/1-XXX | MIL-R-83248, Class 1 (the document cancelled in 2001, its /1 sheet in 2008, when DoD pointed future purchases to SAE AS3209, boss sizes to AS3208); today AMS7276, and SAE later issued AS83248/1 (2014), which DoD adopted on 2025-11-03 | Fluorocarbon (FKM), 75 Shore A | High-temperature fluids | AS568 dash |
| M83248/2-XXX | MIL-R-83248, Class 2 (the document cancelled in 2001, its /2 sheet in 2004); today AMS7259, with SAE AS3581 and SAE AS83248/2 named in the records | Fluorocarbon (FKM), 90 Shore A | High-temperature, high-pressure | AS568 dash |
| AS3209-XXX | AMS 7276 | Fluorocarbon (FKM), 75 Shore A | A common aerospace Viton® callout | AS568 dash |
| M25988/1-XXX | MIL-DTL-25988/1, Active; the base MIL-DTL-25988D has been inactive for new design since 2025-06-11 and is in part superseded by SAE AMS-R-25988 | Fluorosilicone, 70 Shore A | Fuel systems | AS568 dash |
| MS28778-XXX | AMS-P-5510 | Nitrile, 90 Shore A | Straight-thread boss fittings | boss series, its own numbering |
The mil-spec O-ring chart lists the specifications, and the M83248 and AMS7276 guides cover two common fluorocarbon callouts. Note the Parker handbook's own caution: a specification's temperature limits do not necessarily represent the operating limits of the seal.
How do I find the dash number of an O-ring I have?
Measure the inside diameter and the cross-section of an unused ring with calipers, taking the cross-section in two places. The cross-section alone narrows it to one family; the inside diameter picks the number off the chart. A metric callout with a cross-section of 1.78, 2.62, 3.53, 5.33 or 6.99 mm is an AS568 size expressed in millimeters. Sizes between the standard steps are custom or spliced rings, and a ring that measures oversize is usually a used one that has swollen or taken a set (see the compression set guide). Send the two measurements with your quote request and we identify the size.
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Sources
- Parker O-Ring Handbook ORD 5700, section 2, read 2026-09-14
- Parker O-Ring Handbook ORD 5700, Tables 8-3 and 8-4 (page 8-4, 8-5; the compound pairings), read 2026-09-24
- AMS 7276
- AMS-P-5510
- AMS-R-25988
- AS3209
- AS568
- Precision Polymer Engineering, ISO 3601 O-ring Sizes reference table, read 2026-09-06 and 2026-09-28
- ISO 6149
- MIL-H-5606
- MIL-P-25732
- MIL-PRF-25732
- MIL-R-83248
- MS28775
- MS28778
- DLA ASSIST: MIL-DTL-25988/1 (ident 18214; Active, validation notice 2026-04-21), read 2026-09-24, and MIL-DTL-25988D Notice 1 (2025-06-11: inactive for new design, in part superseded by SAE-AMS-R-25988), read 2026-09-28
- DLA ASSIST: MIL-R-83248 (ident 33573, cancelled 2001-06-21, superseded by SAE-AMS7276, SAE-AMS7259, SAE-AMS3216 and SAE-AMS3218), read 2026-09-25; MIL-R-83248/1 (cancelled 2008-07-24), read 2026-09-24, and its cancellation notice (Notice 5), read 2026-09-29; SAE-AS83248/1 (adopted 2025-11-03), read 2026-09-25; MIL-R-83248/2 (cancelled 2004-01-22, superseded by SAE-AS3581 and SAE-AMS7259) and SAE-AS83248/2 (adopted 2025-11-03), read 2026-09-28; MIL-PRF-25732 (Inactive, revision E, 2024), read 2026-09-24
- SAE Mobilus current-revision record for AS83248/1A (issued December 2014), read 2026-09-27
Frequently asked questions
Does the dash number tell me the material?
No. AS568 defines dimensions only. The material comes from the compound code or the material specification in front of the dash number.
What is the difference between AS568-214 and 2-214?
None in size. 2-214 is Parker's catalog prefix for the AS568-214 size; other makers use their own prefixes with the same three digits.
Is AS568B still current?
The revision letter has moved on; the current document is SAE AS568F. Drawings that say AS568A or AS568B call out the same dash sizes, which is why old prints still work.
Why is there no -100 or -200 size?
The families do not use every number. The 3/32 in family starts at -102 and the 1/8 in family at -201; the unused numbers are simply not sizes.
Are the -9xx boss seals in the metric standards?
No. ISO 3601-1 folded in the AS568 general-purpose sizes but excludes the boss-seal series (-901 to -932); metric fitting O-rings are covered by ISO 6149.
How much does the tolerance change across the chart?
It grows with the inside diameter. A -006 is 0.114 in inside diameter plus or minus 0.005 in, a -214 is 0.984 in plus or minus 0.010 in, a -349 at 4.475 in carries plus or minus 0.030 in, and a -395 at 25.94 in carries plus or minus 0.12 in. The cross-section tolerance is steadier: plus or minus 0.003 in on the 0xx and 1xx sections, 0.004 in on 2xx, 0.005 in on 3xx and 0.006 in on 4xx.
What are the -001, -002 and -003 sizes?
Three sizes below the 0xx cord with their own cross-sections: -001 is 0.029 in inside diameter with a 0.040 in section, -002 has a 0.050 in section and -003 a 0.060 in section. They have no ISO 3601 metric counterpart, so a metric drawing cannot call them out by equivalent.
What does a -9xx dash number look like in dimensions?
The 20 boss gaskets carry sections that match the tube-fitting port rather than the five general-purpose cords: -901 is 0.185 in inside diameter with a 0.056 in section, -910 is 0.755 in with a 0.097 in section, and -932 is 2.337 in with a 0.118 in section. The last two digits follow the tube size, so a -910 seals a 5/8 in port and a -932 a 2 in port.
Related guides
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