AS568-309 O-Ring — 0.412 × 0.210 in
The -309 is a 300 series size: a 0.210 in cord on a 0.412 in inside diameter, which is 5.33 mm on 10.46 mm. Its outside diameter is 0.832 in (21.13 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) | 0.412 | 10.46 |
| Cross-section d₂ (cord) | 0.210 | 5.33 |
| Outside diameter (reference) | 0.832 | 21.13 |
| Inside diameter tolerance | ±0.005 | ±0.13 |
| Cross-section tolerance | ±0.005 | ±0.13 |
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 AS568-309 is a dimension
The -309 is a lookup key. Its first digit — 3 — puts it in the 300 Series family, and the remaining two digits are its position inside that family counted upwards by inside diameter, which makes it number 1 of the 87 sizes here. Neither digit group carries a size: the -309 is 0.412 in on a 0.210 in cord, and a dash number one away from it can differ by a hundredth of an inch or by a quarter of one.