I had a career in rubber R&D, engineering and manufacturing. In my last job I had design control over o-rings and other static seals purchased by the corp. My motto was "Take me to your leaker".
Not throwing rocks but if anyone can measure an o-ring accurately enough so it will seal, my hat is off to you
. We and the o-ring supplier used high dollar optical or laser based non-contact measurement systems. Non-contact because when you put a caliper or mic on an o-ring or any other piece of rubber it moves.
Common o-ring cross section diameters are: .070", .103", .139", .210"...etc. When designing an o-ring seal joint you typically went for about 20% compression on the o-ring cross section. So for a .103 that is about .020" of squeeze on the rubber. Get down to 15% and it will probably leak. Get over 25% and the o-ring will probably split. And that is not much room for error.
After fighting o-ring leak problems and doing post mortems for warranty claims for five years if all I had was a caliper and mic to measure the damn things I would have quit my job
O-rings size standards are by Aerospace Standard AS568 which is now controlled by the SAE. Japan and Germany have their versions as well.
You will run across the term o-ring "dash sizes". AS568 defines each o-ring with a 3 digit number preceded by a - (dash) with an example being the -119 (dash 119). That o-ring has an inside diameter of .924 and cross section diameter of .103. The dash size is size only, each size is available in several material types.
Here is a decent reference on o-ring sizes and materials:
http://www.applerubber.com/src/pdf/as568-standard-size-o-rings.pdf
When installing an o-ring best to remember a couple of things: (1) Lube the o-ring and do not let it get twisted or pinched. (2) In any collision between rubber and sharp metal, the rubber will lose the fight.
