Well, I have a couple of gizmos that are dial bore gages. Not the type with three plungers, these have measuring arms. There is a lever to compress them to go into the bore.
On the arms are small points, and the dial reads the distance across the points, complete with cosine error, I think. (They are best used as comparators.)
One of mine goes down to a fairly small size.
Read the inside of the groove diameter with one, and note the reading.
Get your calipers or mic, get the arms between the anvils, and adjust to read same on the dial of the bore gage. The mic is now set to the groove diameter.
The points do have to fit in the groove, and not let the arms bottom out on the main ID. But it works.
Eh..... late edit....
Somehow I missed Toolbert's link.... that is what mine are, less the carbide tips, and with somewhat different design details. Yes, they are in fact "Intertest".
The smaller one goes from 0.20 to 0.60, so the range under 0.5 is available.
[ 12-05-2005, 11:57 PM: Message edited by: J Tiers ]