[Gooserider]

I am not an expert on O-rings, so you will need to double check this - I am certainly open to corrections.

There are two numbers in the size of an O-ring, one is the diameter of the rubber the ring is made out of, and that is a function of the groove dimension. (if designing a block, there is a spec for groove size in the hole spacing sticky in the design forum.) The second number is the diameter of the circle formed by the ring itself. I believe it's the average between the OD and ID, but I could be wrong.

If the O-ring groove isn't circular, then one needs to find the length of the groove, (either by measurement or calculation) then calculate the equivalent diameter of a circle with the same length, then get an O-ring with the same or slightly smaller diameter.
(ie if length = 3.14" then get a 1" diameter O-ring) O-rings can easily be shaped to fit into irregular grooves. They can be stretched SLIGHTLY to do this, but don't like being squished into a shorter length.

If designing a block, it is necessary to design the groove so that it has a length that is the same as an available part. While it is possible to custom make O-rings in any size, this is generally considered a less than optimal approach.

(BTW, if I needed to find the size of an unknown irregular shaped O-ring, groove, I would probably do it by getting a peice of non-stretchy string approx the right diameter and wrapping it into the groove, cutting it to the right length and measuring the result. This would probably be easier, faster and more acurate than any calculation)

Gooserider