Hole center spacing gage

[RetiredFAE]

Great idea and write up Marv!

Made on like it a couple of decades ago, and found another use for it as well.....when mounting something on a wall where the object to be mounted has keyhole slots on the back (and didn't come with a paper mounting template) and is to be mounted over a couple of protruding screw holes, I use the tool by setting it in the large or small section of the keyhole, whichever is easier, then lock it down.

Place it on the wall, with a small pocket level across the top, push in a bit, the cone points mark the spots for drilling and installing the screws.
Object is then easily hung and level.

If it won't be visible, I put a small mark on the top edge of the object, at its very center, and a small mark on the wall at the center between the two screw heads.
Lining up the two small marks makes it really easy to get the object onto the screw heads on the first try.

V/r
Regards,
Steve

[Mook]

With no flats you can zero the caliper on one diameter for direct reading. Or do the simple mental subtraction if working in metric.