O-ring size chart (AS568)

How AS568 dash numbers work

SAE AS568 specifies the inside diameters, cross-sections, tolerances and dash numbers of O-rings for sealing and for straight thread tube fitting boss gaskets. In the standard sizes, the first digit of the dash number tells you the cross-section (also called W or thickness) and the last two count up through the inside diameters.

• -001 to -050: 0.070 in (1.78 mm), except -001, -002 and -003 at 0.040, 0.050 and 0.060 in.
• -102 to -178: 0.103 in (2.62 mm).
• -201 to -284: 0.139 in (3.53 mm).
• -309 to -395: 0.210 in (5.33 mm).
• -425 to -475: 0.275 in (6.99 mm).

That is 349 standard sizes, from -001 at 0.029 in ID to -475 at 25.94 in ID. The -901 to -932 sizes (Parker's 3- series) are meant for sealing straight thread tube fittings in a boss and are not in this chart.

Nominal size and actual size

Each ring has a nominal size, written as fractional ID x OD x cross-section, and an actual size with tolerances. Parker defines a nominal dimension as the nearest fractional equivalent of the actual decimal dimension. Take -214:

• Nominal: 1 x 1-1/4 x 1/8 in
• Actual ID: 0.984 in plus or minus 0.010 in (24.99 plus or minus 0.25 mm)
• Actual cross-section: 0.139 in plus or minus 0.004 in (3.53 plus or minus 0.10 mm)

In every row of this chart the nominal OD equals the nominal ID plus twice the nominal cross-section, and in all rows but -005 the actual ID is below the nominal. Parker's gland design example for a 1.000 in rod picks the -214 because its actual ID, 0.984 in, is slightly smaller than the rod. Design grooves from the seal maker's gland tables rather than from the ring alone.

Tolerances grow with size

Cross-section tolerance is fixed for each series: plus or minus 0.003 in for the 0.070 and 0.103 in sections, 0.004 in for 0.139 in, 0.005 in for 0.210 in and 0.006 in for 0.275 in. ID tolerance grows with diameter, from plus or minus 0.004 in on the smallest rings to plus or minus 0.120 in on the -475. The metric columns are Parker's published conversions, which are rounded and not always exact arithmetic.

The dimensions are AS568A values for standard-shrinkage (AN) compounds, as tabulated by Parker. Parker notes that compounds with other shrinkage rates produce slightly different dimensions and tolerances, and its compound shrinkage table places most of its fluorocarbon (FKM) compounds and many of its silicones outside the AN class. Check the seal maker's data for those if the fit is tight.

Identifying an unknown O-ring

1. Measure the cross-section and compare it with the five standard sections above.
2. Measure the inside diameter with the ring lying flat.
3. Find that cross-section in the chart and pick the dash number whose ID matches within its tolerance.

A ring that has been in service may not measure true. Parker lists swelling and shrinkage from the sealed fluid among the changes to avoid, and describes compression set as a ring permanently deformed into a flat-sided oval. Measure the groove and the mating bore or shaft as well. If the old ring was metric, it may be an ISO 3601-1 size rather than AS568; Parker's handbook lists ISO 3601-1 Series A and Series G sizes separately.

How the chart was checked

All 349 rows come from Parker's O-Ring Handbook ORD 5700, Table 9-1. They were compared by script against Global O-Ring and Seal's AS568 chart and Precision Polymer Engineering's AS568 table. Three typographical errors in the Parker table were corrected to the values in Global's chart: the ID tolerance of -201 (printed 0.055, correct 0.005 in), the ID tolerance of -209 (0.009 in) and the nominal ID of -227 (2-1/8 in). Precision Polymer's table also gives 0.005 and 0.009 in for the two tolerances. Elsewhere it differs from the other two on 14 sizes, mostly by 0.001 in or less, such as -011 at 0.302 in instead of 0.301 in; the largest gap is -449, at 9.983 in against 9.975 in.