There are THREE crucial dimensions to measure when trying to find the right size of O Ring. They are
Outer Diameter (OD)
If you know these dimensions, the material and durometer (hardness) you need, you're all set!
So let's go through them all, and figure out how to measure your current O-Ring to get a replacement, or how to measure your machine, if you don't have an old O-Ring to go off of.
CROSS SECTION: This is the width of one side of the O-Ring. Since O-Rings are circular, this means that if you measure the height while it's lying flat on a table, that is also the cross section. Standard size O-Rings have cross sections that come in 1/32", 1/16", 3/32", 1/8", 3/16", and 1/4", but there are special sizes in standard, and Metric sizes as well. If you're looking at the machine but no O-Ring, this is the width of the groove that the O-Ring fits in (but be careful! Some machines were built to hold 1 or 2 Back-Up rings in addition to the O-Ring).
OUTER DIAMETER (OD): The OD is the total width of the O-Ring, from one outside edge to the other. If you're looking at the groove where the O-Ring sits, it might be the diameter outside of the groove (but, again, be careful, because that's not always true!)
INNER DIAMETER (ID): The ID is the measurement of the interior of the O-Ring, from one inside wall to the opposite inside wall. If you're looking at the groove where the O-Ring fits, it's the diameter of the groove.
COMING SOON: We'll discuss how to decide which material you need for your O-Ring.