This is an original made-in-USA Blake co-ax indicator, but beware of Copy Cats!
Inexpensive versions of the Co-Axial indicator are made in China. Just like the "Rolex" watches you can buy for $30 from any street vendor at Downtown, Any City, USA, they're outright frauds. The made-in-China Co-Axial indicator is of fair quality but a shameful copy. All we can do is strongly urge you to purchase the genuine article—made in the USA—shown on this page.
What is the smallest hole that can be centered?
The standard feeler has a 1/8" ball at the end. However, we have specially made feelers with a 1/16" ball which can be found on [page 110](http://www.longislandindicator.com/p110.html). The smallest hole, in theory, would be 1/16" if you use this feeler.
You are welcome to grind down the standard feeler so that you can work with smaller holes. The ball doesn't have to be round since only one spot touches the surface as you rotate the gage. Another solution, and perhaps a better one, is to put a pin into the hole, and then center it from the outside using the OD feelers.
Additionally, we have the following statement from the manufacturer:
"The Blake Co-Ax indicator is a centering device. It requires a reading of both sides of the bore (or boss), in each axis, to arrive at center. Its sole function is to find center. It is never to be used as a measuring instrument.
You can not certify the Co-Ax on a surface plate. It must be put in the spindle of the machine in which it would normally be used and indicate a precision ring or plug. Then remove it and use a .0001" indicator to check that you are on center.
The Co-Ax indicator is made to be used under power. When checking the Co-Ax position with a test indicator the spindle should be rotated manually for both indicators due to the fact that spindle torque under power on some machines will give a different reading for center."