The AS568 standard is the most widely referenced O-ring sizing system in North America and is used globally in aerospace, automotive, hydraulics, and industrial equipment. Originally developed by the Society of Automotive Engineers (SAE) and maintained as part of the SAE Aerospace Material Specifications family, the AS568 system defines 400+ dash sizes across five cross-sectional series. This guide provides the technical details engineers need to specify, procure, and inspect AS568 O-rings correctly.
Short Answer: AS568 Series Summary
| Series | CS (inch) | CS (mm) | Dash Range | Typical Application |
|---|---|---|---|---|
| 100 series (miniature) | 0.070 in | 1.78 mm | −006 to −050 | Aerospace, instrumentation, small-bore fittings |
| 200 series (industrial) | 0.103 in | 2.62 mm | −102 to −178 | General hydraulics, pneumatics, automotive |
| 300 series (heavy industrial) | 0.139 in | 3.53 mm | −210 to −284 | Heavy machinery, large hydraulics |
| 400 series (large) | 0.210 in | 5.33 mm | −309 to −395 | Large flanges, pipelines |
| 500 series (extra large) | 0.275 in | 6.99 mm | −425 to −475; −901 to −932 | Very large flanges, shipbuilding |
Total standard sizes: 369 nominal sizes across all five series (excluding dash numbers −001 to −005 which are non-standard).
History and Scope of AS568
The AS568 standard was first published in the 1950s to eliminate the proliferation of non-interchangeable O-ring sizes in U.S. military and aerospace equipment. Before standardization, each OEM used proprietary O-ring dimensions — a practice that created supply chain fragility and prevented spare parts interchangeability across equipment.
Current status: AS568 is maintained by SAE International. The standard defines dimensions and tolerances only — it does not specify material or compound requirements. Material specifications for specific applications are called out in companion SAE AMS documents:
- AMS 3216: Nitrile O-rings (NBR) for aerospace
- AMS 3217: Silicone O-rings (VMQ) for aerospace
- AMS-R-83485: Fluorocarbon (FKM) O-rings for aircraft fuel and oil systems
- AMS 7276: EPDM O-rings for aircraft systems
- MIL-PRF-83461: Aerospace elastomer specification (cross-reference)
While AS568 is an inch-based standard, it is cross-referenced to ISO 3601-1, allowing global sourcing from manufacturers who produce to both standards simultaneously.
Dash Number Nomenclature
A complete AS568 O-ring specification includes:
- The prefix "AS568" (or formerly "MS28775" in military applications)
- A dash number identifying the size
- A material specification (added by the engineer, not in AS568 itself)
- Hardness (Shore A)
Example designation: > AS568-214 | FKM 75 Shore A | AMS-R-83485
This designates: AS568 standard, dash number −214 (ID = 0.984 inch / 24.99 mm, CS = 0.139 inch / 3.53 mm), fluorocarbon rubber, 75 Shore A hardness, to the aerospace FKM specification.
Dash number structure (approximate):
- −001 to −050: Miniature sizes (non-standard and standard, all with CS = 0.070 inch)
- −102 to −178: 200 series (CS = 0.103 inch)
- −210 to −284: 300 series (CS = 0.139 inch)
- −309 to −395: 400 series (CS = 0.210 inch)
- −425 to −475: Transition sizes (CS = 0.210 inch or 0.275 inch depending on specific dash)
- −901 to −932: 500 series (CS = 0.275 inch)
The Five Cross-Sectional Series
100/Miniature Series (CS = 0.070 inch / 1.78 mm)
- Dash numbers: −006 to −050 (standard); −001 to −005 are non-standard
- Applications: Aerospace fittings, fuel system instrumentation, medical devices, pneumatic miniature valves, watch seals
- Design note: This CS is also the base CS for all 200-series O-rings but at smaller IDs. Dash −006 through −050 cover IDs from 0.070 to 1.983 inches; the 200 series covers the same CS with additional IDs in the 0.301–5.609 inch range.
200 Series (CS = 0.103 inch / 2.62 mm)
- Dash numbers: −102 to −178
- ID range: Approximately 0.239 inch (6.07 mm) to 5.609 inch (142.47 mm)
- Applications: General-purpose hydraulics, pneumatics, automotive systems, plumbing, instrumentation
- Notes: The most commonly used series in industrial equipment. Standard hydraulic cylinder and valve applications are well-covered by −110 to −178.
300 Series (CS = 0.139 inch / 3.53 mm)
- Dash numbers: −210 to −284
- ID range: Approximately 0.734 inch (18.64 mm) to 6.859 inch (174.22 mm)
- Applications: Heavy machinery, large hydraulic cylinders, process equipment, flanged joints
- Notes: The second most common series after the 200 series. Provides better pressure resistance than 200 series at the same groove depth; suitable for 70–100 bar dynamic service without backup rings (70 Shore A compound).
400 Series (CS = 0.210 inch / 5.33 mm)
- Dash numbers: −309 to −395
- ID range: Approximately 1.475 inch (37.47 mm) to 24.940 inch (633.47 mm)
- Applications: Large-diameter flanges, pipelines, heavy construction equipment, compressor cylinder covers
- Notes: Used where larger seal cross-section is needed for pressure resistance and conformability on rough or wide-face flange surfaces. Groove width requirements are larger — verify machining capabilities for the wider groove before specifying this series.
500 Series (CS = 0.275 inch / 6.99 mm)
- Dash numbers: −425 to −475; −901 to −932
- ID range: 4.475 inch (113.67 mm) to 25.975 inch (659.77 mm)
- Applications: Very large flanges, shipbuilding, off-road equipment, wind turbine main bearings, large valve bonnets
- Notes: The largest standard cross-section. Often used in split-gland configurations or where high groove depth can be accommodated. Groove machining for 6.99 mm CS requires wider cuts than standard tooling in some CNC configurations — verify before designing.
Common AS568 Sizes and Dimensions
Miniature / 100 Series (CS = 0.070 in / 1.78 mm)
| Dash No. | ID (inch) | ID (mm) | OD (inch) | OD (mm) |
|---|---|---|---|---|
| −006 | 0.070 | 1.78 | 0.210 | 5.33 |
| −008 | 0.090 | 2.29 | 0.230 | 5.84 |
| −010 | 0.112 | 2.84 | 0.252 | 6.40 |
| −012 | 0.301 | 7.65 | 0.441 | 11.20 |
| −014 | 0.364 | 9.25 | 0.504 | 12.80 |
| −020 | 0.674 | 17.12 | 0.814 | 20.68 |
| −030 | 1.171 | 29.74 | 1.311 | 33.30 |
| −040 | 1.669 | 42.39 | 1.809 | 45.95 |
| −050 | 1.983 | 50.37 | 2.123 | 53.92 |
200 Series (CS = 0.103 in / 2.62 mm)
| Dash No. | ID (inch) | ID (mm) | OD (inch) | OD (mm) |
|---|---|---|---|---|
| −110 | 0.362 | 9.19 | 0.568 | 14.43 |
| −112 | 0.487 | 12.37 | 0.693 | 17.60 |
| −114 | 0.612 | 15.54 | 0.818 | 20.78 |
| −116 | 0.737 | 18.72 | 0.943 | 23.95 |
| −118 | 0.862 | 21.89 | 1.068 | 27.13 |
| −120 | 0.987 | 25.07 | 1.193 | 30.30 |
| −125 | 1.237 | 31.42 | 1.443 | 36.65 |
| −130 | 1.487 | 37.77 | 1.693 | 43.00 |
| −150 | 2.487 | 63.17 | 2.693 | 68.40 |
| −178 | 5.609 | 142.47 | 5.815 | 147.70 |
300 Series (CS = 0.139 in / 3.53 mm)
| Dash No. | ID (inch) | ID (mm) | OD (inch) | OD (mm) |
|---|---|---|---|---|
| −210 | 0.734 | 18.64 | 1.012 | 25.70 |
| −211 | 0.796 | 20.22 | 1.074 | 27.28 |
| −212 | 0.859 | 21.82 | 1.137 | 28.88 |
| −213 | 0.921 | 23.39 | 1.199 | 30.45 |
| −214 | 0.984 | 24.99 | 1.262 | 32.05 |
| −215 | 1.046 | 26.57 | 1.324 | 33.63 |
| −216 | 1.109 | 28.17 | 1.387 | 35.23 |
| −217 | 1.171 | 29.74 | 1.449 | 36.80 |
| −218 | 1.234 | 31.34 | 1.512 | 38.40 |
| −219 | 1.296 | 32.92 | 1.574 | 39.98 |
| −220 | 1.359 | 34.52 | 1.637 | 41.58 |
| −222 | 1.484 | 37.69 | 1.762 | 44.75 |
| −250 | 3.484 | 88.49 | 3.762 | 95.55 |
| −284 | 6.859 | 174.22 | 7.137 | 181.28 |
400 Series (CS = 0.210 in / 5.33 mm)
| Dash No. | ID (inch) | ID (mm) | OD (inch) | OD (mm) |
|---|---|---|---|---|
| −309 | 1.475 | 37.47 | 1.895 | 48.13 |
| −312 | 1.663 | 42.24 | 2.083 | 52.91 |
| −325 | 2.350 | 59.69 | 2.770 | 70.35 |
| −350 | 4.975 | 126.37 | 5.395 | 137.03 |
| −395 | 24.940 | 633.47 | 25.360 | 644.13 |
500 Series (CS = 0.275 in / 6.99 mm)
| Dash No. | ID (inch) | ID (mm) | OD (inch) | OD (mm) |
|---|---|---|---|---|
| −425 | 4.475 | 113.67 | 5.025 | 127.64 |
| −426 | 4.600 | 116.84 | 5.150 | 130.81 |
| −428 | 4.850 | 123.19 | 5.400 | 137.16 |
| −430 | 5.100 | 129.54 | 5.650 | 143.51 |
| −901 | 1.100 | 27.94 | 1.650 | 41.91 |
| −910 | 5.975 | 151.76 | 6.525 | 165.74 |
| −920 | 10.975 | 278.77 | 11.525 | 292.74 |
| −932 | 25.975 | 659.77 | 26.525 | 673.73 |
Dimensional Tolerances
AS568 specifies two tolerance classes:
Class 1 (Standard): The normal manufacturing tolerance for commercial and industrial applications. Achieved by standard compression molding without precision tooling.
Class 2 (Precision): Tighter tolerances for aerospace, high-precision hydraulics, and applications where seal compression must be controlled very accurately. Requires precision mold tooling and tighter process controls.
AS568 Class 1 vs Class 2 Tolerance Comparison
| CS Series | Class 1 CS Tolerance | Class 2 CS Tolerance | Class 1 ID Tolerance (at mid-series) | Class 2 ID Tolerance (at mid-series) |
|---|---|---|---|---|
| 100 series (1.78 mm) | ±0.08 mm (±0.003 in) | ±0.05 mm (±0.002 in) | ±0.18 mm (±0.007 in) | ±0.10 mm (±0.004 in) |
| 200 series (2.62 mm) | ±0.08 mm (±0.003 in) | ±0.05 mm (±0.002 in) | ±0.25 mm (±0.010 in) | ±0.13 mm (±0.005 in) |
| 300 series (3.53 mm) | ±0.10 mm (±0.004 in) | ±0.08 mm (±0.003 in) | ±0.36 mm (±0.014 in) | ±0.18 mm (±0.007 in) |
| 400 series (5.33 mm) | ±0.13 mm (±0.005 in) | ±0.10 mm (±0.004 in) | ±0.61 mm (±0.024 in) | ±0.30 mm (±0.012 in) |
| 500 series (6.99 mm) | ±0.15 mm (±0.006 in) | ±0.13 mm (±0.005 in) | ±0.69 mm (±0.027 in) | ±0.36 mm (±0.014 in) |
When to specify Class 2: Precision hydraulic servo systems, proportional valves, aerospace fuel systems, and applications where the tolerance band on O-ring CS directly affects compression rate calculation. Standard industrial applications use Class 1.
ID Tolerance: Why It Increases With Size
AS568 ID tolerance is not a fixed percentage — it is specified in absolute units that increase as ID increases. The engineering rationale: larger O-rings are measured and handled over larger distances, making dimensional variation proportionally harder to eliminate through mold precision alone. Thermal expansion of the mold itself becomes significant at large diameters. The absolute tolerance values are set so that the relative tolerance (tolerance / ID) stays within a band that is acceptable for standard gland design calculations.
Relationship to ISO 3601-1
ISO 3601-1 is the international metric standard for O-rings. While AS568 and ISO 3601-1 are not identical, there is substantial overlap. Many global manufacturers produce O-rings conforming to both standards simultaneously.
Key differences:
- Primary units: AS568 uses inches; ISO 3601-1 uses millimeters
- Tolerance grades: ISO 3601-1 uses Grade N (normal) and Grade S (special/precision), roughly analogous to AS568 Class 1 and Class 2
- Size range: ISO 3601-1 includes metric sizes not found in AS568, particularly for European machinery (e.g., 2.5 mm, 4.0 mm, 4.5 mm CS not common in AS568)
- Identification system: ISO uses ID × CS notation (e.g., 25.00 × 3.53 mm) rather than dash numbers
AS568 to ISO 3601-1 Cross-Reference (Common Sizes)
| AS568 Dash | AS568 ID × CS (inch) | Nearest ISO 3601 Size (mm) | Dimensional Difference |
|---|---|---|---|
| −006 | 0.070 × 0.070 | 1.80 × 1.80 | Essentially identical |
| −110 | 0.362 × 0.103 | 9.19 × 2.62 | < 0.1 mm |
| −112 | 0.487 × 0.103 | 12.37 × 2.62 | < 0.1 mm |
| −210 | 0.734 × 0.139 | 18.77 × 3.53 | 0.13 mm ID difference |
| −213 | 0.921 × 0.139 | 23.39 × 3.53 | < 0.1 mm |
| −214 | 0.984 × 0.139 | 25.00 × 3.53 | 0.01 mm — essentially identical |
| −219 | 1.296 × 0.139 | 32.92 × 3.53 | < 0.1 mm |
| −222 | 1.484 × 0.139 | 37.69 × 3.53 | < 0.1 mm |
| −325 | 2.350 × 0.210 | 59.69 × 5.33 | < 0.1 mm |
| −425 | 4.475 × 0.275 | 113.67 × 6.99 | < 0.1 mm |
For most engineering purposes, the dimensional differences between AS568 and the nearest ISO size are negligible (< 0.5 mm). However, in critical aerospace or high-precision hydraulic systems, always specify the exact standard and verify supplier conformance — do not assume interchangeability without dimensional verification.
Gland Design Reference for AS568 Sizes
AS568 is paired with SAE ARP 1231 (aerospace gland design reference) and SAE ARP 5681 (updated static and dynamic gland dimensions) for complete gland specification.
Dynamic (Reciprocating) Groove Dimensions — Standard AS568 Guidance
| CS (inch) / CS (mm) | Groove Depth Range (inch) | Groove Depth Range (mm) | Groove Width (inch) | Groove Width (mm) |
|---|---|---|---|---|
| 0.070 / 1.78 mm | 0.058–0.064 | 1.47–1.63 | 0.093–0.098 | 2.36–2.49 |
| 0.103 / 2.62 mm | 0.087–0.093 | 2.21–2.36 | 0.137–0.143 | 3.48–3.63 |
| 0.139 / 3.53 mm | 0.118–0.125 | 3.00–3.18 | 0.185–0.191 | 4.70–4.85 |
| 0.210 / 5.33 mm | 0.181–0.190 | 4.60–4.83 | 0.280–0.288 | 7.11–7.32 |
| 0.275 / 6.99 mm | 0.238–0.248 | 6.05–6.30 | 0.365–0.375 | 9.27–9.53 |
Squeeze calculation: Squeeze (%) = (CS − Groove Depth) / CS × 100. For 3.53 mm CS with groove depth 3.09 mm: squeeze = (3.53 − 3.09) / 3.53 × 100 = 12.5% — within the recommended 10–15% for dynamic service.
Static Groove Dimensions (Higher Squeeze)
Static grooves are designed for 15–25% squeeze rather than the 10–15% used for dynamic seals. This provides greater contact stress for reliable sealing against pressure without the friction concern present in dynamic applications. Groove depths for static service are approximately 0.75–0.85 × CS (versus 0.84–0.90 × CS for dynamic grooves).
Visual and Dimensional Acceptance Criteria
AS568-conforming O-rings are inspected to the following criteria (per AS568 and supporting SAE/ASTM documents):
Dimensional:
- ID within the tolerance band for the specific dash number and Class (1 or 2)
- CS within the tolerance band for the specific CS series and Class
- Measured at three points around the circumference for CS; optical comparator or cone gauge for ID
Visual (per SAE AS568):
- Flash (material at parting line): not to exceed 0.003 inch (0.08 mm) in height for standard commercial grade; 0.005 inch (0.13 mm) maximum in any localized area
- No porosity, blisters, or voids visible at the surface
- No surface cracks, tears, or cuts
- No foreign inclusions visible at the surface
- Mold marks (flow lines): acceptable if they do not exceed the flash height limit
Batch testing for critical applications:
- Hardness per ASTM D2240 (Shore A): ±5 Shore A from nominal
- Tensile strength and elongation per ASTM D412
- Compression set per ASTM D395 Method B
- Fluid swell per ASTM D471 (if required by application specification)
Material and Hardness Specification
AS568 defines sizes and tolerances but does not specify material. The material callout is added by the design engineer based on the application. Common material-hardness combinations:
| Material | Hardness | Typical Application | AMS Specification |
|---|---|---|---|
| NBR | 70 Shore A | General hydraulic, pneumatic, fuel-compatible | AMS 3216 |
| NBR | 90 Shore A | High-pressure, abrasive environments | AMS 3216 |
| FKM | 75 Shore A | High-temperature, fuel-resistant | AMS-R-83485 |
| FKM | 90 Shore A | High-temperature, high-pressure static | AMS-R-83485 |
| EPDM | 70 Shore A | Water, steam, phosphate ester (Skydrol) | AMS 7276 |
| VMQ (Silicone) | 70 Shore A | Low-temperature, food-grade | AMS 3217 |
| HNBR | 70 Shore A | Elevated temperature, ozone resistance | (Commercial spec) |
| PTFE | — | Chemical-resistant; spring-energized seals | (Commercial spec) |
For aerospace applications, the AMS material specification is called out alongside the AS568 dash number. For non-aerospace industrial applications, the material is specified by compound description (e.g., "NBR, 70 Shore A, per ASTM D2000 5AA510") or by the designer's in-house compound specification.
FAQ
Q1: What does the dash number mean in AS568?
The dash number is a size identifier, not a dimension in itself. For example, AS568-214 identifies an O-ring with inside diameter 0.984 inch (24.99 mm) and cross-section 0.139 inch (3.53 mm). The dash number allows engineers to specify an exact size with a single code rather than quoting both ID and CS. Dash numbers do not imply anything about material, hardness, or application — those are specified separately.
Q2: Can I use an AS568 O-ring in a metric gland designed for ISO 3601-1?
Usually yes for common sizes where the dimensional difference is < 0.5 mm. AS568 and ISO 3601-1 share many near-identical dimensions (e.g., AS568-214 at 24.99 × 3.53 mm vs ISO 25.00 × 3.53 mm — 0.01 mm difference). However, always verify gland fill ratio and squeeze percentage using actual dimensions rather than assuming interchangeability. For precision applications (servo hydraulics, aerospace), specify the exact standard and verify supplier conformance.
Q3: What is the largest AS568 size available?
The largest standard AS568 dash number is −932 in the 500 series, with nominal ID of 25.975 inches (659.77 mm) and cross-section 0.275 inch (6.99 mm). Larger sizes are available as custom cord-spliced or vulcanized rings in any diameter. For IDs above 660 mm, cord-splice is the standard manufacturing method — no compression mold tooling exists for these sizes.
Q4: Are AS568 tolerances tight enough for aerospace use?
Standard AS568 Class 1 tolerances are acceptable for most commercial and industrial aerospace applications. For high-precision hydraulic servo systems and critical fuel-wetted seals, AS568 Class 2 (tighter) tolerances should be specified. Some aerospace applications may impose additional requirements beyond AS568 — verify the relevant AMS specification (AMS-R-83485 for FKM, AMS 3216 for NBR) for material-specific tolerances and inspection requirements.
Q5: How do I specify a custom O-ring size that does not match an AS568 dash number?
Specify inside diameter and cross-sectional diameter in inches or millimeters with explicit tolerances, along with material, hardness, and tolerance class. Example: "O-ring, ID = 45.00 ± 0.25 mm, CS = 4.00 ± 0.10 mm, FKM 75 Shore A, black, ISO 3601-1 Grade N." For dimensions close to a standard AS568 dash number, consider whether the gland can be adjusted to accept the standard size — avoiding custom tooling reduces cost and lead time.
Q6: What is the difference between AS568 and AMS-R-83485?
AS568 defines the physical dimensions and tolerances of the O-ring. AMS-R-83485 defines the material specification for FKM O-rings used in aircraft fuel and oil systems — including compound chemistry requirements, physical property requirements, batch testing requirements, and traceability documentation. An aerospace O-ring is typically specified to both: AS568-214 defines the size; AMS-R-83485 defines the material. Both must be satisfied simultaneously.
Q7: Does AS568 cover O-ring materials or just dimensions?
AS568 covers dimensions and tolerances only — it does not specify elastomer type, compound formulation, or physical properties. Material requirements are specified separately by the design engineer using: an AMS specification (for aerospace), an ASTM D2000 line call (for industrial automotive), a manufacturer's in-house specification, or a verbal description of material type and hardness. The most complete specification approach for critical applications is: AS568 dash number (for dimensions) + AMS compound specification (for material) + required test reports.
Q8: How does AS568 relate to the SAE ARP 1231 gland design standard?
AS568 defines O-ring dimensions. SAE ARP 1231 (Aerospace Recommended Practice 1231) defines the corresponding groove (gland) dimensions for AS568 O-rings in aerospace applications — groove depth, width, corner radius, surface finish, and lead-in chamfer. For non-aerospace applications, equivalent groove design guidance is in SAE ARP 5681. Engineers specifying AS568 O-rings for new gland designs should reference the appropriate ARP for gland dimensions rather than designing grooves from scratch.
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