About the AS 568 A standard for O-rings

The AS 568 A standard for O-rings is produced by the Society of Automotive Engineers AS (Aerospace Standard). The AS 568 A specification describes 369 O-rings. Each size is designated with a dash number and can be described using two values: the inside diameter and diameter of the cross-section. Except for the -9## O-rings, all American inch O-rings can be categorized into seven cross-sections and 119 inside diameters.

The nominal size for any given American inch o-ring, however, is different than the actual physical dimension of the o-ring. As an example, consider the -001 o-ring. Nominally, it is described as having a cross-section of ¹⁄₃₂ inch. The decimal equivalent of ¹⁄₃₂ inch is 0.03125 inch. However, the actual cross-section is 0.04 inch. Why the discrepancy? The nominal sizing of the American inch system is meant to describe the size of the groove in which the O-ring resides. Since all O-rings must be squeezed by about 10% to 15% in order to seal properly, the extra size of the O-ring handles the needed squeeze.