---
type: "o-ring"
dash_number: "255"
designation: "AS568-255"
cs_series: "201-284"
cs_series_name: "AS568 1/8 in series"
id_in: 5.609
id_mm: 142.47
cs_in: 0.139
cs_mm: 3.53
od_in: 5.887
od_mm: 149.53
id_tol_in: 0.035
cs_tol_in: 0.0039
standard: "SAE AS568; MIL-G-5514G (public domain, inactive for new design)"
verified: "August 1, 2026"
url: https://shopmath.org/o-rings/as568-255
---

# AS568-255 O-Ring Size — ID, Cross-Section and OD

An AS568-255 o-ring has an inside diameter of 5.6090 in (142.470 mm) and a 0.1390 in (3.530 mm) cross-section, for an outside diameter of 5.8870 in (149.530 mm). It belongs to the AS568 1/8 in series band.

- **Dash number**: 255
- **Cross-section series**: AS568 1/8 in series
- **Inside diameter**: 5.6090 in (142.470 mm)
- **Cross-section**: 0.1390 in (3.530 mm)
- **Outside diameter**: 5.8870 in (149.530 mm)
- **Standard**: SAE AS568; MIL-G-5514G (public domain, inactive for new design)

## AS568-255 Dimensions and Tolerances

| Dimension | Inch | Millimetre |
| --- | --- | --- |
| Inside diameter (ID) | 5.6090 | 142.470 |
| Cross-section (CS) | 0.1390 | 3.530 |
| Outside diameter (OD) | 5.8870 | 149.530 |
| ID tolerance | ±0.0350 | ±0.890 |
| CS tolerance | ±0.0039 | ±0.100 |

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

## Gland Design for AS568-255

Static gland dimensions for the 0.139 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 | 20–30% | 0.0278 – 0.0417 | 0.0973 – 0.1112 | 2.471 – 2.824 |

_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 AS568-255

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) |
| --- | --- | --- | --- | --- |
| [AS568-358](https://shopmath.org/o-rings/as568-358.md) | 5.6000 | 142.240 | 0.2100 | 6.0200 |
| [AS568-434](https://shopmath.org/o-rings/as568-434.md) | 5.6000 | 142.240 | 0.2750 | 6.1500 |
| [AS568-359](https://shopmath.org/o-rings/as568-359.md) | 5.7250 | 145.420 | 0.2100 | 6.1450 |
| [AS568-435](https://shopmath.org/o-rings/as568-435.md) | 5.7250 | 145.420 | 0.2750 | 6.2750 |
| [AS568-161](https://shopmath.org/o-rings/as568-161.md) | 5.4870 | 139.370 | 0.1030 | 5.6930 |
| [AS568-256](https://shopmath.org/o-rings/as568-256.md) | 5.7340 | 145.640 | 0.1390 | 6.0120 |

## Sources and Method for AS568-255

### 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 AS568-255 O-Ring

### What size is an AS568-255 o-ring?

AS568-255 has an inside diameter of 5.6090 in (142.470 mm) and a cross-section of 0.1390 in (3.530 mm), giving an outside diameter of 5.8870 in (149.530 mm).

### What is the o-ring size tolerance on AS568-255?

The AS568 tolerance on AS568-255 is ±0.0350 in (±0.890 mm) on the inside diameter and ±0.0039 in (±0.100 mm) on the cross-section.

### What groove depth does an AS568-255 o-ring need?

For a static face seal, an 0.139 in cross-section needs a gland depth of 0.0973 to 0.1112 in (2.471 to 2.824 mm), which puts the ring at 20–30% squeeze.

### What o-ring is closest in size to AS568-255?

The nearest AS568 sizes by inside diameter are AS568-358 (5.6000 in ID), AS568-434 (5.6000 in ID), AS568-359 (5.7250 in ID).

## Related pages

- Previous dash number: [AS568-254](https://shopmath.org/o-rings/as568-254.md) — 5.4840 in ID
- Next dash number: [AS568-256](https://shopmath.org/o-rings/as568-256.md) — 5.7340 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/as568-255) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*