I picked up a Co-Ax off ebuy a few years ago, and I'm getting my money's worth out of it. Bought the 3" inside feeler, and was stumped on why it wouldn't pick up on a 1.375 bore...
Manly-man adventures deleted. The secret to adjusting the swept diameter is to pivot the feeler outward. The phillips screw just above the feeler socket is the pivot for the feeler assembly. You can loosen it up or tighten it with the phillips screw / acorn nut. Duh.
To answer the question of what the Co-Ax measures, the dial says (Quote).0005" AXIS
Does that " stand for inches or minutes?
The dial graduation is divided such that each division represents .0005" (0,01mm) of axis offset (deviation from true location) when the feeler is tracing a diameter of two inches. This will vary slightly with feeler angle. - manufacturer's instructions