Inside Measurements by Overlapping Tapes
Adding two tape readings that meet at a round number to read a wall-to-wall span exactly.
Zero Digital Calipers on a Hole to Read Center-to-Center Directly
Measure one hole, press zero, then measure across the outer edges of both. The caliper has already subtracted the hole diameter, so the number on the display is the center-to-center distance.
bolt hole spacing, distance between screw holes, bolt pattern measurement, drill hole layout
The Problem
Center-to-center between two holes is the number you need to lay out a matching part, and it's the one number calipers can't measure directly — there's nothing at a hole's center to put a jaw against. The usual answer is to measure edge to edge and do arithmetic: add a diameter to the inside measurement, or subtract one from the outside. Both work, and both mean holding a reading in your head while you move a tool, which is where the mistake gets made.
The Technique
Make the caliper do the subtraction.
- Measure the diameter of one hole with the inside jaws.
- With the jaws still in the hole, press the zero button. The caliper now reads zero at that width, and every subsequent reading is the true distance minus one hole diameter.
- Measure across the two holes from the far edge of one to the far edge of the other.
- Read the display. That's your center-to-center distance.
The outside-to-outside span of two equal holes is the center distance plus exactly one diameter — so zeroing out a diameter first leaves the center distance on the screen with nothing to add or subtract.
Pro tip: This assumes both holes are the same size. On a pair of different diameters the arithmetic no longer cancels, and you're back to measuring edge to edge and working it out — zeroing on one of the two would quietly give you a wrong answer rather than an obviously wrong one.
Pro tip: Zero the caliper back out when you're finished. A tool left with an offset dialled in reads convincingly wrong for the next job, and there's nothing on the display to tell you it's been zeroed against something.
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