Are you talking about spring calipers or sliding calipers, Dave? If we were
to sit down and try it together I could demonstrate repeatable accuracy in a
2" ring of +/- 0.002" with spring calipers; probably half that with good
instruments. An expert can do even better. But if you're talking about
sliding-scale calipers, they're next to useless for measuring the inside of
a ring.

--
Ed Huntress

Ed,
I'm speaking of direct measuring sliding calipers, I have vernier,
dial and electronic.

I think they are greatly over-rated.
Dave