AS568-205 O-Ring — 0.421 × 0.139 in
The -205 is a 200 series size: a 0.139 in cord on a 0.421 in inside diameter, which is 3.53 mm on 10.69 mm. Its outside diameter is 0.699 in (17.75 mm), derived as ID + 2 × cord.
Dimensionsthe two cells the standard fixes, and one derived value
| Dimension | Inches (AS568) | Millimetres (converted) |
|---|---|---|
| Inside diameter d₁ (ID) | 0.421 | 10.69 |
| Cross-section d₂ (cord) | 0.139 | 3.53 |
| Outside diameter (reference) | 0.699 | 17.75 |
| Inside diameter tolerance | ±0.005 | ±0.13 |
| Cross-section tolerance | ±0.004 | ±0.1 |
Inches are the standard’s own units; millimetre values are conversions — inch × 25.4, rounded to two decimals — and not the ISO 3601-1 metric sizes. Outside diameter is a reference value derived as ID + 2 × cross-section; the two diameters the standard fixes are the inside diameter and the cord.
Reading the designationnothing in AS568-205 is a dimension
The -205 is a lookup key. Its first digit — 2 — puts it in the 200 Series family, and the remaining two digits are its position inside that family counted upwards by inside diameter, which makes it number 5 of the 84 sizes here. Neither digit group carries a size: the -205 is 0.421 in on a 0.139 in cord, and a dash number one away from it can differ by a hundredth of an inch or by a quarter of one.