O-ring size is typically specified according to AS 568A, the Aerospace Standard Uniform Dash Numbering System developed by the Society of Automotive Engineers (SAE). Per AS 568A, the size of each O-ring can be noted in terms of its inside diameter (I.D.) and width (W). (Width is also referred to as cross-section, or CS.) Each figure also includes a tolerance, or allowable deviation (plus and minus). Measurements are often given in both inches and millimeters.

Size listings may include two sets of figures: actual size and nominal size. The actual size is the exact size of the O-ring in decimal dimensions (inches or millimeters), including tolerances. The nominal size is the approximate size of the O-ring in fractional dimensions (inches, e.g. 1 1/8") and is generally included strictly for reference purposes. In addition to the inside diameter and width figures, a nominal listing may also include an outside diameter (O.D.) listing. Keep in mind that the width of an O-ring is one-half the difference between its O.D. and I.D. (see Figure 72).

The O-ring size that is best suited for a particular project depends on the needs of the seal. Keep in mind that cross-section determines sealability. Larger cross-sections typically provide more stability and allow more tolerance, but they also create more seal friction. Smaller sections generate less friction but are less stable and allow less tolerance. Regardless of the application, it is imperative that the cross-section be greater than the height of the gland into which it will be installed. Squeeze applied to the seal will compress the O-ring. The combination of compression and system pressure will cause the O-ring to block the diametral clearance gap and prevent leakage. The effects of variances in cross-section size are shown in Table 36.

DIMENSIONS MAIN PAGE