AS568-389 O-Ring — 19.955 × 0.210 in
The -389 is a 300 series size: a 0.210 in cord on a 19.955 in inside diameter, which is 5.33 mm on 506.86 mm. Its outside diameter is 20.375 in (517.53 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) | 19.955 | 506.86 |
| Cross-section d₂ (cord) | 0.210 | 5.33 |
| Outside diameter (reference) | 20.375 | 517.53 |
| Inside diameter tolerance | ±0.095 | ±2.41 |
| 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 AS568-389 is a dimension
The -389 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 81 of the 87 sizes here. Neither digit group carries a size: the -389 is 19.955 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.
The same inside diameter, in another seriesa matching bore does not make two rings interchangeable
| Series | Dash | Cord (in) | Cord (mm) | ID tol. |
|---|---|---|---|---|
| 300 Series | -389 | 0.210 | 5.33 | ±0.095 |
| 400 Series | -469 | 0.275 | 6.99 | blank |
These rings share an inside diameter and nothing else. The cords differ, and so do the inside diameter tolerances on some of them — the tolerance is a per-size figure in the standard, not a function of the diameter, so it is printed rather than derived.