AS568-121 O-Ring — 1.049 × 0.103 in
The -121 is a 100 series size: a 0.103 in cord on a 1.049 in inside diameter, which is 2.62 mm on 26.64 mm. Its outside diameter is 1.255 in (31.88 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) | 1.049 | 26.64 |
| Cross-section d₂ (cord) | 0.103 | 2.62 |
| Outside diameter (reference) | 1.255 | 31.88 |
| Inside diameter tolerance | ±0.010 | ±0.25 |
| Cross-section tolerance | ±0.003 | ±0.08 |
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 AS568-121 is a dimension
The -121 is a lookup key. Its first digit — 1 — puts it in the 100 Series family, and the remaining two digits are its position inside that family counted upwards by inside diameter, which makes it number 20 of the 77 sizes here. Neither digit group carries a size: the -121 is 1.049 in on a 0.103 in cord, and a dash number one away from it can differ by a hundredth of an inch or by a quarter of one.