The O-Rings are available in standard metric sizes (cross sections are in 0.5 millimeter and inside diameters are in 1 millimeter increments) and in standard inch sizes. The inche sizes is the Aerospace Standard 568 published by the Society of Automotive Engineers. The inch sizes are designated by dash number as shown on the size chart AS568 or AS568A. AS568 or AS 568 was used to be called ARP568. For instance, an O-Rings with an in- 010.