AS568-427 O-Ring — 4.725 × 0.275 in
The -427 is a 400 series size: a 0.275 in cord on a 4.725 in inside diameter, which is 6.99 mm on 120.02 mm. Its outside diameter is 5.275 in (133.99 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) | 4.725 | 120.02 |
| Cross-section d₂ (cord) | 0.275 | 6.99 |
| Outside diameter (reference) | 5.275 | 133.99 |
| Inside diameter tolerance | ±0.033 | ±0.84 |
| Cross-section tolerance | ±0.006 | ±0.15 |
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 AS568-427 is a dimension
The -427 is a lookup key. Its first digit — 4 — puts it in the 400 Series family, and the remaining two digits are its position inside that family counted upwards by inside diameter, which makes it number 3 of the 51 sizes here. Neither digit group carries a size: the -427 is 4.725 in on a 0.275 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. |
|---|---|---|---|---|
| 400 Series | -427 | 0.275 | 6.99 | ±0.033 |
| 300 Series | -351 | 0.210 | 5.33 | ±0.030 |
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.