O-ring measuring calipers.
Fullsize image: https://diqn32j8nouaz.cloudfront.net...s_fullsize.jpg
https://diqn32j8nouaz.cloudfront.net...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
