Buna-N O-Ring Dash Sizes: AS568-376 and -222 Explained

What Dash Size Means for a Buna-N O-Ring

The AS568 standard assigns a dash number to each o-ring size. That number links directly to the inner diameter (ID) and cross-section (CS) of the seal. For a Buna-N nitrile o-ring, the dash size is the quickest way to confirm you have the right fit without measuring every time.

This guide compares two common sizes: AS568-376 and AS568-222. Both are 70 durometer Shore A, black Buna-N, but their dimensions suit completely different applications.

Size Comparison at a Glance

• 0.21" CS X 11.475" ID Buna-N Nitrile O-Rings | AS568-376 | Dash 376 | 70 Durometer | Black (30 / Pkg.): AS568-376: 11.475" (291.46 mm) inner diameter, 0.21" (5.33 mm) cross-section. This is a large-diameter seal for big flanges, pipe connections, or heavy equipment where the groove is wide enough to accept a 0.21" cross-section.

  

• 0.139" CS X 1.484" ID Buna-N Nitrile O-Rings | AS568-222 | Dash 222 | 70 Durometer | Black (500 / Pkg.): AS568-222: 1.484" (37.69 mm) inner diameter, 0.139" (3.53 mm) cross-section. A much smaller seal suited to compact hydraulic ports, small-diameter fittings, or applications where space for the groove is limited.

  

Material: Buna-N Nitrile at 70 Durometer

Both o-rings use Buna-N (NBR) with a 70 Shore A hardness. This is the most common durometer for general-purpose sealing because it balances flexibility with resistance to extrusion. The temperature range for both is -40°F to +250°F (-40°C to +121°C), so they work in moderate heat and cold.

Buna-N resists petroleum-based oils, fuels, and hydraulic fluids well. If your application involves strong acids, ozone, or extreme temperatures outside that range, you would need a different material — but for most industrial sealing jobs, 70 durometer Buna-N is a reliable starting point.

Choosing Between the Two Sizes

Your choice depends entirely on the gland dimensions of your assembly. Measure the groove inner diameter and depth, then match them to the o-ring ID and cross-section. The AS568-376 fits large bores (roughly 11.5" diameter), while the AS568-222 fits much smaller bores (about 1.5" diameter).

Pack quantity is another practical difference. The AS568-376 comes in a 30-piece package, reflecting its larger size and lower typical replacement frequency. The AS568-222 comes in a 500-piece package, which makes sense for high-volume maintenance or production runs where small o-rings are used in quantity.

Pack Sizes and Cost Context

• 0.21" CS X 11.475" ID Buna-N Nitrile O-Rings | AS568-376 | Dash 376 | 70 Durometer | Black (30 / Pkg.): 30 o-rings per package at a catalog price of USD 43.13. Large o-rings are less frequently replaced, so a smaller pack keeps inventory manageable and per-piece cost reasonable for one-off repairs.
• 0.139" CS X 1.484" ID Buna-N Nitrile O-Rings | AS568-222 | Dash 222 | 70 Durometer | Black (500 / Pkg.): 500 o-rings per package at a catalog price of USD 45.36. Small o-rings are often used in multiples. A 500-count pack gives a low per-unit cost for ongoing maintenance or assembly work.

What the Catalog Does Not Tell You

The supplied information confirms dimensions, material, temperature range, and durometer, but it does not include compression set data or dynamic sealing limits. For high-pressure or reciprocating applications, you may need additional manufacturer data to confirm the o-ring will hold up over many cycles.

Also, while both o-rings are black 70 durometer Buna-N, the catalog does not specify whether the compound meets specific industry standards such as ASTM D2000 or SAE J200. If your project requires a certified material grade, check with the supplier before ordering.