O-ring measuring calipers.
Fullsize image: https://s3-us-west-1.amazonaws.com/h...s_fullsize.jpg
https://s3-us-west-1.amazonaws.com/h...g_calipers.jpg
Previously:
https://www.homemadetools.net/forum/...684#post147799
https://www.homemadetools.net/forum/...526#post135969
https://www.homemadetools.net/forum/...343#post122020
