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AS568-251 O-Ring Size
200 series · 1/8" cross section · General purpose, cylinders
Inside diameter × cross section
129.77 × 3.53 mm
5.109″ × 0.139″ ·
outside diameter 136.83 mm (5.387″)
Dimensions
| Dimension | Inches | Millimetres |
|---|---|---|
| Inside diameter (ID) | 5.109″ | 129.77 mm |
| Cross section (CS) | 0.139″ | 3.53 mm |
| Outside diameter (OD) | 5.387″ | 136.83 mm |
Outside diameter is not measured, it is derived: OD = ID + 2 × CS.
If your measured OD disagrees with this, trust the ID and cross section — measuring
across the outside of a flexible ring is the least reliable of the three.
If you measured a used one
A ring pulled out of service will measure smaller than 129.77 mm, because compression set is permanent. If your measurement came in a little under this and the cross section roughly matches, -251 is very likely still the correct part. Ordering the size your worn ring measures is how the same failure repeats. More on this →
Nearby sizes in the 200 series
These share the same 1/8" cross section, so if the thickness is right but the diameter is off, your ring is probably one of these.
| Dash | ID (mm) | ID (in) | OD (mm) |
|---|---|---|---|
| -250 | 126.59 | 4.984″ | 133.65 |
| -252 | 132.94 | 5.234″ | 140.0 |
| -249 | 123.42 | 4.859″ | 130.48 |
| -253 | 136.12 | 5.359″ | 143.18 |