Hello firstpass, there is a machinists tool that I believe was referred to as an external caliper. There are many different kinds of calipers, the one in particular that I am thinking of has two curved arms, at one end they are somewhat wide, this is the end where they have a hole drilled through them and one of the arms is attached to the other arm with a tensioning screw. Each arm is curved and tapers to almost a point from the wide end where they pivot off of one another to the other end where they will touch if they are completely closed. You can open these arms up and pass a round object between the points and close them so that when the round item passes between them you are able to then use a tape or rule to determine the diameter of the round object. It is likely that you could also use this tool to determine the amount of out-of-round of something as well. There are versions of this tool that can be used to check relatively small items(0 to say 2" dia.) and also larger versions to check possibly 72" or better dia. Hopefully you were able to follow what I was describing here. Sorry I don't have a picture. Best regards, aevald
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