In article , a
says...

Bore gauges are a must have if you do a lot of accurate i/d work. I have three
sets. The main one for engine bores is a Mercer 2" - 6" which reads to a tenth
of a thou. It does that fairly repeatably too.

I bought a couple of Federal dial bore gages at work
for some finicky bits. They are worth every penny.

Best is if you can set them with ring gages, I've
been making do with micrometers. Not right but
they still show variation at the 0.0001 level.

Jim

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