OpenSpecEngineering

Sizes, materials and seal types for anyone specifying or replacing an O-ring. Look up an AS568 dash number for its full dimensions, or enter a measured ring and find the dash number it matches. Then check which elastomer suits the fluid and temperature, and whether an O-ring is the right seal at all.

What the page covers

• AS568 Standard O-Ring Sizing: which dash-number range goes with which cross-section, from SAE AS568D [1].
• AS568 Size Lookup: every size in AS568D [1], including the -901 to -932 boss gaskets for straight thread tube fittings. By dash number it gives ID, W, OD, the ID and W limits, and the reference volume. By measured size it ranks the six nearest sizes.
• O-Ring Material Comparison: temperature range, hardness and fluid compatibility for six common materials, after the Parker handbook [2] and the Marco guide [3].
• Seal Types Beyond O-Rings: where lip, face, quad-ring and gasket seals fit better [2].

Reading the sizes

ID is the inside diameter, W the cross-section and OD the outside diameter, with OD = ID + 2W. The dash number is a code, not a dimension. Its hundreds digit sets the cross-section, so -224 is a 0.139″ ring. AS568D prints only minimum and maximum limits. The page shows the midpoint as the nominal and half the range as the ± tolerance. “Within limits” means your measurement falls inside the printed limits, checked in the unit you entered.

Limits and common mistakes

• A used ring has swollen, shrunk or taken a set. Measure the gland when you can, or confirm a doubtful match against the groove.
• Enter the cross-section whenever you know it. ID alone can match a ring of the wrong thickness.
• The page doesn’t design grooves, squeeze or stretch. Use the Parker handbook [2] for that.
• The material rows are general. Temperature range and compatibility vary by compound, so check the supplier’s data for the exact fluid.

Worked example

A hydraulic cylinder end cap leaks, and its O-ring has no part number. Calipers read 43.9 mm ID and 3.5 mm cross-section. Find the replacement.

1. In the lookup, choose Look up by Measured size and Units Millimeters.
2. Enter Inside diameter (ID) 43.9 and Cross-section (W), optional 3.5. The page shows: Best match: AS568-224 (within limits)
   ID 44.04 ± 0.38 mm · W 3.53 mm · your ID is −0.14 mm from nominal The neighbors -223 and -225 are outside limits.
3. Choose Look up by Dash number and Dash number -224. The page shows: ID 1.734 ± 0.015 in (44.04 mm)
   W 0.139 ± 0.004 in (3.53 mm)
   OD 2.012 in (51.10 mm)
   Volume 0.0893 in³ (1.463 cm³)
4. For hydraulic oil, the material table points to Nitrile/Buna-N (NBR), good to 250 °F.

Check: 1.734 + 2 × 0.139 = 2.012″ OD. A torus of mean diameter 1.734 + 0.139 = 1.873″ has volume π² × 1.873 × 0.139² / 4 = 0.0893 in³, the same as the page.