AS568-128 O-ring dimensions, tolerances and equivalents
Short answer: an AS568-128 O-ring has an inside diameter of 1.487 in ± 0.012 (37.77 mm ± 0.30) and a cross-section of 0.103 in ± 0.003 (2.62 mm ± 0.08). Parker calls the same size 2-128. The nominal inside diameter is 1-1/2 in, the nominal outside diameter is 1-11/16 in and the nominal cross-section is 3/32 in.
The part that catches buyers is that AS568-222 and AS568-325 also have a 1-1/2 in nominal inside diameter, within 0.3 mm of the -128, but they have a 3.53 mm and a 5.33 mm cross-section. An O-ring is identified by inside diameter and cross-section together. This guide gives the dimensions from the Parker O-Ring Handbook, which tabulates the AS568 sizes, and shows how to avoid ordering the wrong ring.
AS568-128 at a glance
| Item | Inch | Millimetre |
|---|---|---|
| Designation | AS568-128 (Parker 2-128) | Same ring |
| Nominal inside diameter | 1-1/2 | 38.1 |
| Nominal outside diameter | 1-11/16 | 42.9 |
| Nominal cross-section | 3/32 | 2.38 |
| Actual inside diameter | 1.487 ± .012 | 37.77 ± 0.30 |
| Actual cross-section | .103 ± .003 | 2.62 ± 0.08 |
Source: Parker O-Ring Handbook ORD 5700, Parker Series 2-XXX sizes table (AS568A sizes). Millimetre nominal values are the inch fractions converted at 25.4 mm per inch.
The neighbouring dash numbers
Dash numbers in the 1xx series step up in inside diameter while the cross-section stays at 0.103 in (2.62 mm). A ring one dash number away is a different inside diameter, not a tolerance variant of the -128.
| AS568 size | Actual inside diameter (in) | Actual inside diameter (mm) | Cross-section |
|---|---|---|---|
| -127 | 1.424 ± .012 | 36.17 ± 0.30 | .103 in / 2.62 mm |
| -128 | 1.487 ± .012 | 37.77 ± 0.30 | .103 ± .003 in / 2.62 ± 0.08 mm |
| -129 | 1.549 ± .015 | 39.34 ± 0.38 | .103 in / 2.62 mm |
Source: Parker O-Ring Handbook ORD 5700, Series 2-XXX sizes table.
Same inside diameter, different ring: -128, -222 and -325
The three standard series with a 1-1/2 in nominal inside diameter are the 1xx series (2.62 mm cross-section), the 2xx series (3.53 mm) and the 3xx series (5.33 mm). Their actual inside diameters differ by less than the tolerance of the larger rings, so a caliper reading of the inside diameter alone cannot separate them. The cross-section can.
| AS568 size | Nominal ID / OD / CS (in) | Actual ID | Actual cross-section |
|---|---|---|---|
| -128 | 1-1/2 / 1-11/16 / 3/32 | 1.487 ± .012 in (37.77 ± 0.30 mm) | .103 ± .003 in (2.62 ± 0.08 mm) |
| -222 | 1-1/2 / 1-3/4 / 1/8 | 1.484 ± .015 in (37.69 ± 0.38 mm) | .139 ± .004 in (3.53 ± 0.10 mm) |
| -325 | 1-1/2 / 1-7/8 / 3/16 | 1.475 ± .015 in (37.47 ± 0.38 mm) | .210 ± .005 in (5.33 ± 0.13 mm) |
The -128 inside diameter is 0.08 mm larger than the -222 and 0.30 mm larger than the -325. Source: Parker O-Ring Handbook ORD 5700, Series 2-XXX sizes table.
What the dash number tells you
AS568 groups sizes by series, and the first digit of the dash number sets the cross-section. The ranges below are the series MST lists for FKM and FFKM O-rings.
| Series | Dash numbers listed by MST | Cross-section |
|---|---|---|
| 0xx | -006 to -030 | 1.78 mm |
| 1xx | -110 to -138 | 2.62 mm (.103 in) |
| 2xx | -210 to -234 | 3.53 mm (.139 in) |
| 3xx | -325 to -350 | 5.33 mm (.210 in) |
Source: SAE AS568 size standard as tabulated in the Parker O-Ring Handbook; dash-number ranges are the sizes in the MST catalog.
How to identify an unknown ring: measure both numbers
- Measure the cross-section first, at several points around the ring, and take the average. The series follows from it: about 2.62 mm is a 1xx ring, about 3.53 mm is a 2xx ring, about 5.33 mm is a 3xx ring.
- Measure the free inside diameter with the ring lying relaxed on a flat surface. A ring that is stretched, twisted or swollen from service reads wrong.
- Find the row in the Parker size table where both the inside diameter and the cross-section fall inside that row’s tolerance.
- If two rows match on inside diameter, the cross-section decides. For a ring near 37.7 mm inside diameter, 2.62 mm is -128, 3.53 mm is -222 and 5.33 mm is -325.
- Record the material separately. The dash number sets the size only; the compound (FKM75, FFKM and so on) is a second line on the order.
Equivalents: same size in other naming
Parker prefixes its AS568 sizes with the series number: the -128 is Parker 2-128 and the -222 is Parker 2-222. In metric terms the -128 is 37.77 mm inside diameter by 2.62 mm cross-section. These are the same ring described three ways. A ring that merely has a similar inside diameter, such as the -222 at 37.69 mm, is a different size because its cross-section is 3.53 mm.
The material is a separate choice from the size. For the -128, MST lists an FKM75 request-for-quote item and an FFKM request-for-quote item. Parker gives a general FKM service range of about −26 to +205 °C; FFKM is the choice for plasma or aggressive chemistry, and the FFKM selection guide below lists the grades by process.
Request the right size
The catalog pages below are request-for-quote items. State the AS568 dash number, the material and the quantity, and add the process step and temperature if the ring goes into a process tool. MST is a distributor and technical sourcing partner; the quote states the manufacturer, grade and supply channel.
Frequently asked questions
What are the dimensions of an AS568-128 O-ring?
Inside diameter 1.487 in ± .012 (37.77 mm ± 0.30) and cross-section .103 in ± .003 (2.62 mm ± 0.08). The nominal size is 1-1/2 in inside diameter, 1-11/16 in outside diameter and 3/32 in cross-section.
Is Parker 2-128 the same as AS568-128?
Yes. Parker lists its 2-XXX sizes against the AS568 dash numbers, so 2-128 is the AS568-128 ring.
Can I use an AS568-222 in place of an AS568-128?
No. The inside diameters are 0.08 mm apart (37.69 mm and 37.77 mm), but the -222 has a 3.53 mm cross-section against 2.62 mm for the -128. The cross-sections differ by 0.91 mm, so the rings fill a groove differently.
How do I tell an AS568-128 from an AS568-325?
Measure the cross-section. The -128 is 2.62 mm and the -325 is 5.33 mm, although both have a nominal 1-1/2 in inside diameter and actual inside diameters of 37.77 mm and 37.47 mm.
What is the metric size of an AS568-128?
37.77 mm ± 0.30 inside diameter by 2.62 mm ± 0.08 cross-section, from the inch values in the Parker table.
Does MST offer the AS568-128 in FKM and FFKM?
MST lists AS568-128 request-for-quote items in FKM75 and FFKM. Send the quantity and any process details through the quote form; the quote states the manufacturer, grade and supply channel.
Request a quote
Send the AS568 dash number, material and quantity. MST, as distributor and technical sourcing partner, replies with the manufacturer, grade and supply channel for the AS568-128 or the neighbouring sizes.
Request a quoteRelated pages
Sources
- Parker O-Ring Handbook ORD 5700 (Series 2-XXX sizes, AS568A)
- SAE AS568, Aerospace Size Standard for O-Rings
Values are taken from the manufacturer documents above, checked 2026-10-27. Request a quote for current price and availability. MST is a distributor; products are made by the manufacturers named.