Choosing the Right Bore Gauge

June 10, 2019 9:46 am

Machining is the only field where boring can actually be considered entertaining.

Jokes aside, boring is a frequent operation in machine shops as holemaking applications are pretty common, especially in the automotive or gunsmithing sectors.

Since you need a specific set of tools to perform boring operations, hole measurement also requires specific tooling. These tools, commonly known as bore gauges (or gages), come in a wide variety of types and sizes and are used to measure the inside diameter of a hole or cylinder.

Although they are all designed to deliver very precise measurements, some of them are more accurate than others, especially when it comes to repeated measurements. This is mostly due to the fact that some tools are easier to position than others and therefore require more skills and practice.

Let’s take a look at the different types of tools to identify which ones are the most accurate and efficient.


Telescoping Gauges

Telescoping gages measure the size of the bores by transferring the internal dimension to a remote measuring tool, in most cases a micrometer. They are typically T-shaped, with a head featuring telescopic rods that can retract and extend to fit the hole.

Measuring an inside diameter with this type of tool is fairly simple. After choosing the appropriate size, the head of the gage is inserted in the hole and locked with the locking screw at located at the bottom of the tool. The size of the head is then measured using a device such as a micrometer.



  • No practice needed to fit it into the hole
  • Can be used to measure deep holes
  • Fairly inexpensive when compared to other ID measuring tools



  • Requires a second measuring tool to take the measurement
  • Less accurate than other tools



Starrett S579GZ Self Centering Telescoping Gauge With 2 Telescoping Arms 


Inside Calipers

Inside calipers, also called spring-joint calipers, consist of two legs, a spring, and an adjusting nut. The friction of the screw and adjusting nut counteracts the tension of the spring which determines the position of the legs.

Just like telescoping gages, old school inside calipers don’t measure the diameter directly and they need to be used in combination with a measuring tool such as a ruler or micrometer. Modern digital models, however, are now equipped with a display to read the measurement.

Although they are a valuable tool, inside calipers are not used as much nowadays as they were in the past.



  • Fairly affordable



  • Not as accurate as other ID measuring tools
  • Requires “skilled” hands to position it correctly in the hole
  • Not suitable for measuring deep bores
  • Requires a second measuring tool to take the measurement



Fowler 74-554-730 Digital Caliper


Inside Micrometers

Inside micrometers are accurate, direct measuring tools. They are available in two different types: rod-type (or tubular) and jawed.

Jawed micrometers, similar in shape to calipers, are mostly used to measure small holes.

Rod-type micrometers, characterized by their “pen-like” shape, are more suitable for larger diameters as they are extendible.  They function like regular micrometers.  As the thimble turns, the micrometer expands like a curtain rod. This then extends until each end of the tool is touching the inside of the bore. When this happens, you use the numbering system on the thimble to find your measurement.



  • Directly reads the measurement
  • Very accurate



  • Practice is needed to obtain an accurate measurement as it’s not easy to make sure that the tool is perpendicular to the hole axis
  • Not suitable for deep bores
  • Needs to be calibrated
  • Good models are expensive



Mitutoyo 139-201, 1.5″ – 12″ X .001″ Tublar Inside Micrometer


Dial Bore Gauges

Dial bore gauges are the most used tool for measuring inside diameters and for a good reason. A dial bore gauge is easier to position than any other tool mentioned above and is very accurate. It’s a sensible option if you are going to be measuring bores frequently as the read-out is given on the dial, with no secondary measuring. They are easy to set up, as the read-out will tell you when the tool is perpendicular to the hole axis and they are great for measuring the taper of deep bores.



  • Easy to set up, very little skill required
  • Suitable for deep bores
  • Extremely accurate
  • Adjustable to different diameter sizes
  • Doesn’t require a second measuring tool



  • Needs to be calibrated with every use



Fowler Full Warranty Extender Dial Bore Gage Set, 52-646-400, 1.4-6″ Measuring Range, 0.0005″ 


Three-point Bore Micrometers

By far the easiest and most accurate tool to measure internal diameters. They are self-centering, self-aligning and can be extended for deep holes. What’s the catch? Out of all the tools we recommend above, they are the most expensive.



  • Extremely accurate
  • Easy to position
  • Quick to use and easy to read
  • Suitable for deep holes



  • Analog models require calibration
  • Expensive



Starrett 781XTZ-258 3-Point Contact 2-2 5/8-Inch Range


Who’s the Winner?

Dial bore gauges are a good compromise as they work great with any type of hole, they are not too expensive and are fairly easy to use. As always, however, there’s no clear answer to this question. The right measuring tool is inevitably related to the needs of your shop. How many parts do you have to measure? What do they look like? Do you need to measure deep or shallow bores? What’s the tolerance? How much are you willing to spend for a measuring tool?
Answering all these questions should help you identify the right tool for your application.


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  • Steve says:

    I used top work at a shop that used Air pressure bore gauges that were very acurate

    • Steve says:

      I used these guages which we called air probes, way back in 1978. They were perfect because they would show exactly where your out of roundness was located. We were able to show deviations in the tenths. We would mark the area and allow the electroplating shop onsite to build up the area out of spec. Saved many expensive parts for further machining processes. It’s the only place I ran across air probes.

  • Sean the Dog says:

    Three-point Bore Micrometers (tri-mics, as we called them) will not show out-of-round conditions very well, because they show an average for the location being measured.

    Dial bore gauges give the best picture of what the bore actually looks like at each location being checked (short of a CMM).

  • Harold says:

    Accuracy begins with the operator, nothing is infallible.

  • Air gages ar fine but must have a Super good r.m.s. Self centering Self aligning. 3 jaw are the best.

    • Tornado220M says:

      no that;s not true so long as they are calibrated for the surface finish being measured then they work best.

  • Bob says:

    You can’t beat a plug gage like Deltronic to check the actual usable size of a hole. Tri-mikes require calibrated ring gages to check the low, center, and high ends of the measuring range. They also have a cone that is prone to wear when used in one area. Close tolerance work also requires a temperature controlled environment of 68 degrees F., 20 degrees C.

  • Woodstock says:

    Absolutely ! Air Pressure Gages can measure .00005!

  • Jon says:

    The problem og getting an accurate measurement can lie with the hole at least as much as the gauge. If the hole is well finished (and round and not irregular) the problem itself is easier thna otherwise. The two point gauges you showed do a good job much of the time, but you have to ask “what is the measurement to be used for?”. There is another type of gauge with two almost circular measuring elements forced apart by a tapered wedge, that is also accurate, but it sits on the peaks of any surface imperfections more than the 2 point opposed buttons type in the article, so a practical hole, with a less than perfect finish, can look to be a different size depending on which of these measuring approaches is used. A part of my working life is to measure the bores to fit bushes that I make to accomodate things like the camshafts in classic motorcycles and I usuually end up using more than one method and going for the one that seemed most reproducable when I was doing the measuring. There is anothe method I use a lot, and that is the tapered hole measuring wedges that Starrett (used?) to make. These are very reliable, they can be padded out with parallels to increase their range, and when you use a micrometer to see how far apart the measuring faces are, you quickly get a feeling of confidence. I have done measuring with these to 2 tenths and they delivered OK. Just my views. HTH. Jon

  • ex-Gooserider says:

    Dentists are also into boring – whether it is entertaining depends on whether you are the boreR or the boreE…..

  • Tornado220M says:

    only as long as the conditions the operator is measuring in are satisfactory.

  • LWD says:

    I cannot agree that telescopic gauges are not accurate .if used correctly.
    To use a telescopic gauge lock the gauge in the bore at an angle to the bore then full the handle to be parallel to the bore .The faces of the gauge will move to the precise diameter.Then use a bench mounted micrometer Telescopic gauges will measure accuracy to microns provide you have a good surface finish The telescopic gauges do eventually wear out .
    After 60 years in the trade I have a number of worn out gauges
    These gauges are especially convenient when boring on the lathe , mill or line borer

  • magicniner says:

    You missed out split ball types like Diatest which are very useful in many circumstances.

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