About the UNI standard for O-rings

The UNI standard for O-rings is produced by Ente Nazionale Italiano di Unificazione (Italian Organization for Standardization). The UNI specification describes 428 O-rings. Each size is designated by an OR followed by a three, four, or five digit number.

In contrast to the AS 568 A inch system, the nominal and physical dimensions of metric O-rings match precisely. That is, a 1 mm O-ring will have a cross-section of 1 mm. The space that is being filled must be about 10% to 15% smaller than the cross-section of the O-ring in order for O-ring to seal properly.