How to Read a Bore Gauge

The standard use for bore gauges is to measure the bore in an engine's cylinder head, but the tool's handy nature makes it ideal for measuring the inner diameter of many other holes or bores with depth. You can use a bore gauge to measure the inside of a pipe or the dimensions of a hole cut into a thick board. More expensive bore gauges with dial readouts display bore dimensions directly, but cheaper and more common telescopic or small head bores require the use of another measuring tool to read the final dimensions.

Telescopic or Small Hole Bore Gauges

• 1

  Prepare the instrument for insertion according to the model of the bore gauge. Telescopic bore gauges open wider than the diameter of the hole and shrink into the diameter of the hole. Open small hole bore gauges to a diameter slightly smaller than the bore's.

• 2

  Follow the instructions provided with the gauge to lock the gauge to the size of the bore. Remove the gauge from the bore.

• 3

  Measure the bore gauge's head, or the distance from the end of the its anvil to the end of its stationary stud. Use a micrometer or anvil to measure the head. The value shown by the micrometer or caliper is the size of the bore.

Dial Bore Gauge

• 1

  Zero the dial bore gauge against a calibrated ring of the same relative size as the bore being measured.

• 2

  Insert the head of the gauge into the bore following the tool's operating instructions. Rock the tool back and forth gently in the bore once the gauge is inserted and standing upright.

• 3

  Watch the readout on the dial face as you rock the gauge back and forth. Record the value of the largest deviation away from "0" on the dial face. Record the deviation as a positive number if it falls to the right of "0," and a negative number if it falls to the left of "0."

• 4

  Add or subtract the value of the largest deviation from the calibrated value of the bore gauge. If the gauge was zeroed at 5 inches, and the largest deviation was .007 inch to the right of the "0" on the dial face, then the final measurement of the bore is 5.007 inches. Alternatively, if the largest deviation was .006 inches to the left of the "0" on the dial face of a gauge calibrated to 5 inches, then the final measurement of the bore is 4.994 inches