A dial bore gauge is a special tool, calibrated in .001 inch (.0025 centimeter) or .0001 inch (.00025 centimeter), which is used to accurately measure the inside diameter of a hole, cylinder or pipe. In conjunction with a micrometer, a bore gauge will give you the exact reading of a bore size. A typical bore gauge is comprised of a shaft with a dial indicator at the top and a measuring sled at the base. The measuring sled consists of three guides and an actuating plunger. Dial bore gauges give you quick and accurate readings on the size, less than perfect roundness or wear [source:

Plastic Services].

Here's how to use a dial bore gauge:

  1. Zero the gauge. This is accomplished by measuring across the gauge with an outside micrometer set to the precise bore size. When the zero aligns with the needle you're ready.
  2. Insert the gauge into the bore to the depth you need. Rock it back and forth until it's properly inserted in the pipe.
  3. Look at the reading. This is the lowest reading, which is taken when the gauge is square on the bore, and the indicator needle reverses its direction. It can be either more or less than the zero mark, and will indicate an oversize or undersize bore [source: Tresna Instrument].