Greg Bender

http://www.thisoldtractor.com/gtbender

O-ring sizes - Metric system

I learned much of this information from the Frederick Seal website.

Standards and sizing

The metric system for o-rings is not nearly so organized as the American inch system. There are many thousands of reasonably available sizes. Typically, each country has a range of sizes.

In contrast to the American 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.

With all the available sizes, it can be quite difficult to accurately determine the size of a metric o-ring; especially one that has been used.

Generally available sizes