Gone for diving
Contributor
The big ones in particular are very hard to measure precisely
Don't try to measure the oring other than thickness....
measure the housing, or the bore.
You can use a tape measure across the bore. With care you can do quite accurate work.
Standard orings can be measured by inside or outside diameter... figure out the thickness, then the ID or the OD. Look on an oring chart, an get the dash#
Tip for standard orings, for instance a 1/8 oring will measure (new) a bit over .125"....
Like aprox ..136" the oring compression is built into the oring, unlike metric orings. Which can be a real pain.