---
type: "o-ring"
dash_number: "391"
designation: "AS568-391"
cs_series: "309-395"
cs_series_name: "AS568 3/16 in series"
id_in: 21.955
id_mm: 557.66
cs_in: 0.21
cs_mm: 5.33
od_in: 22.375
od_mm: 568.32
id_tol_in: 0.1
cs_tol_in: 0.0051
standard: "SAE AS568; MIL-G-5514G (public domain, inactive for new design)"
verified: "August 1, 2026"
url: https://shopmath.org/o-rings/as568-391
---

# AS568-391 O-Ring Size — ID, Cross-Section and OD

An AS568-391 o-ring has an inside diameter of 21.9550 in (557.660 mm) and a 0.2100 in (5.330 mm) cross-section, for an outside diameter of 22.3750 in (568.320 mm). It belongs to the AS568 3/16 in series band.

- **Dash number**: 391
- **Cross-section series**: AS568 3/16 in series
- **Inside diameter**: 21.9550 in (557.660 mm)
- **Cross-section**: 0.2100 in (5.330 mm)
- **Outside diameter**: 22.3750 in (568.320 mm)
- **Standard**: SAE AS568; MIL-G-5514G (public domain, inactive for new design)

## AS568-391 Dimensions and Tolerances

| Dimension | Inch | Millimetre |
| --- | --- | --- |
| Inside diameter (ID) | 21.9550 | 557.660 |
| Cross-section (CS) | 0.2100 | 5.330 |
| Outside diameter (OD) | 22.3750 | 568.320 |
| ID tolerance | ±0.1000 | ±2.540 |
| CS tolerance | ±0.0051 | ±0.130 |

_Outside diameter is computed as ID + 2 × CS; AS568 tabulates ID and CS._

## Gland Design for AS568-391

Static gland dimensions for the 0.210 in cross-section. Depth is the groove depth that produces the stated squeeze on a nominal ring.

| Application | Squeeze | Squeeze (in) | Gland depth (in) | Gland depth (mm) |
| --- | --- | --- | --- | --- |
| Static face seal | 21–30% | 0.0441 – 0.0630 | 0.1470 – 0.1659 | 3.734 – 4.214 |

_Gland depth is computed as CS − (CS × squeeze %), then checked against the published design charts. Groove width, corner radius and back-up ring allowances are outside this dataset._

## O-Ring Sizes Closest to AS568-391

Nearest AS568 sizes by inside diameter — the substitutes to check when this one is out of stock.

| Size | ID (in) | ID (mm) | CS (in) | OD (in) |
| --- | --- | --- | --- | --- |
| [AS568-471](https://shopmath.org/o-rings/as568-471.md) | 21.9550 | 557.660 | 0.2750 | 22.5050 |
| [AS568-392](https://shopmath.org/o-rings/as568-392.md) | 22.9400 | 582.680 | 0.2100 | 23.3600 |
| [AS568-472](https://shopmath.org/o-rings/as568-472.md) | 22.9400 | 582.680 | 0.2750 | 23.4900 |
| [AS568-390](https://shopmath.org/o-rings/as568-390.md) | 20.9550 | 532.260 | 0.2100 | 21.3750 |
| [AS568-470](https://shopmath.org/o-rings/as568-470.md) | 20.9550 | 532.260 | 0.2750 | 21.5050 |
| [AS568-393](https://shopmath.org/o-rings/as568-393.md) | 23.9400 | 608.080 | 0.2100 | 24.3600 |

## Sources and Method for AS568-391

### AS568 dimensions

- **Source**: bd_warehouse o-ring_parameters.csv (Apache-2.0) and bckasper3/KasperCalc oring-sizes.js (MIT), agreeing on all 349 sizes; OD computed as ID + 2 x CS
- **Method**: computed+compiled
- **Verified against**: two independent open datasets agreeing on all 349 sizes; fabricated-dataset negative control passed; 21 checks total

### Gland design

- **Source**: MIL-G-5514G (public domain) gland design criteria; squeeze-percentage bands cross-read from Parker O-Ring Handbook ORD-5700 Design Charts 4-2/4-3 (verification only); depth = CS - CS x squeeze%
- **Method**: computed
- **Verified against**: computed depth band brackets the published design-chart band for every series

### Assumptions

- Dimensions are for a nominal, unstretched ring at room temperature. Installed stretch reduces cross-section.
- Gland figures are static-application only and assume a nominal ring; they carry no compound, temperature or pressure assumption.
- Dataset scope: A fabricated-dataset negative control is asserted against on every run. EXCLUDED: groove width, corner radius, diametral clearance, back-up ring widths, surface finish, all dynamic glands, the 900-series, and ISO 3601 metric sizes.

**Data last verified**: August 1, 2026

## Common Questions About the AS568-391 O-Ring

### What size is an AS568-391 o-ring?

AS568-391 has an inside diameter of 21.9550 in (557.660 mm) and a cross-section of 0.2100 in (5.330 mm), giving an outside diameter of 22.3750 in (568.320 mm).

### What is the o-ring size tolerance on AS568-391?

The AS568 tolerance on AS568-391 is ±0.1000 in (±2.540 mm) on the inside diameter and ±0.0051 in (±0.130 mm) on the cross-section.

### What groove depth does an AS568-391 o-ring need?

For a static face seal, an 0.210 in cross-section needs a gland depth of 0.1470 to 0.1659 in (3.734 to 4.214 mm), which puts the ring at 21–30% squeeze.

### What o-ring is closest in size to AS568-391?

The nearest AS568 sizes by inside diameter are AS568-471 (21.9550 in ID), AS568-392 (22.9400 in ID), AS568-472 (22.9400 in ID).

## Related pages

- Previous dash number: [AS568-390](https://shopmath.org/o-rings/as568-390.md) — 20.9550 in ID
- Next dash number: [AS568-392](https://shopmath.org/o-rings/as568-392.md) — 22.9400 in ID
- All o-ring sizes: [AS568 o-ring reference](https://shopmath.org/o-rings.md)
- Full size chart: [AS568 o-ring size chart](https://shopmath.org/charts/o-ring-sizes.md)

---

*Source: [ShopMath](https://shopmath.org/o-rings/as568-391) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*