Most American maintenance people find o-rings very confusing. When dealing with the measurement of o-rings, they assume any divergence from their routine replacement experience must be a "metric" size. Since America's measuring system is primarily inch, foot and yard based, they believe that any "other" measurement has to be a metric size.
How Many Sizes?
There is a bewildering difference between the American inch system for o-rings and the metric system. The American inch system is tightly organized and has 119 inside diameters (ID) and 5 cross sections (c/s), resulting in 349 o-ring combinations recognized as standard sizes, and known as "DASH NUMBERS." The metric system is loosely organized and has few internationally recognized standard sizes. It is estimated that there are roughly 15,000 reasonably available sizes in the metric o-ring range. Each metric country has a range of sizes that represents an abbreviated version of a standard size range for that country.
What's the Difference?
The principal difference between the two measuring systems for o-rings is that inch-sized o-rings have an actual size that is always larger than the nominal size. For example, an o-ring with a 1/8" c/s should measure .125", but actually measures .139". Metric o-rings have an actual size and a nominal size that are the same. For example, a metric o-ring with a nominal size of 3.5mm will actually measure 3.5mm.
Since all o-rings need to be squeezed by approximately 15% of their volume to function properly, the difference between the two systems is how the squeeze is achieved. In the inch system the o-ring groove is roughly equal to the nominal size, so that the groove measures .125" deep, and the o-ring to fit in it measures .139". Under operation, the o-ring will be squeezed into the groove to 1/8" thick. Metric o-ring grooves are just the opposite. They are shallower than the nominal size; so the groove measures 3.1mm deep for a 3.5mm c/s o-ring and achieves the same squeeze.
Both Reach the Goal
Below is a table showing the five o-ring cross sections in the US measuring system known as the AS568A Universal Dash Series and selected metric o-ring cross sections for reciprocating sealing applications: