The Anatomy of a Telescopic Gauge

February 12, 2020 10:15 am

Telescoping gauges, commonly referred to as snap gauges or telescoping bore gauges, are indirect measuring devices used to measure the internal diameter of a bore, hole, groove, slot, etc. This T-shaped tool consists of a handle, two telescopic rods and a locking screw.

telescopic gauge

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There are two styles of telescopic gauges. One type has two plunging telescopic rods. The other type only has one plunging rod, the second rod remains fixed. The telescopic rods are what measure the distance of the bore, with the help of a micrometer. The ends of the rod are the shape of a half ball, called the radius edge. It’s this shape that’s responsible for the telescopic gauge’s accuracy.

How to use telescopic gauges

Using the telescoping gauge is fairly easy. Although to obtain accurate measurements and to develop a good feel for the tool, a little bit of practice is required. Here’s a step-by-step process:

1. Collapse both the measuring heads by rotating the locking screw anticlockwise.

2. Place the gauge into the bore with one head held in place against the wall of the bore. Begin to loosen the screw, but not too much because the rods are spring loaded.

3.Proceed to tilt the gauge so the rods are slightly inclined and tighten the locking screw.

4. Whichever end you lifted a bit above horizontal, force it slowly downward through the bore. As you do this, wiggle the gauge back and forth. This will allow the gauge to find its smallest size on the spring.

5. You are now ready to measure with a calibrated micrometer, since there is no scale on the tool itself.

Check out how machinist and YouTuber Adam Booth achieves accurate measurements using his non-traditional approach with a telescopic gauge.

Are telescope gauges reliable?

It’s typical for different machinists to have different techniques due to the “touchiness” of the telescopic gauges. A very real argument exists among the metalworking community that telescoping gauges are unreliable especially when it comes to repeatability. Once you develop your feel for the tool, telescopic gauges are a respectable option. Just like anything else, achieving successful and repeatable results takes a bit of practice. Since your success is very dependent on your growing feel for the tool, it’s recommended as you get started to double check your readings.

Recommended models

Starrett S579GZ Self Centering Telescoping Gauge With 2 Telescoping Arms Set


With two plunging telescopic rods, this set measures holes, slots and recesses up to 6 inches in diameter and comes with a protective case. These gauges work for measuring both to the tenth and to the thousandth.

Mitutoyo 155-903, 6-piece Telesopic Gage Set



Both legs of the gauge are plunging in this Mitutoyo set. If the type of machining you do calls for long reach, these would be the ones for you. Measures up to 6 inches.

Find other Mitutoyo gauge ranges and set sizes here and here.

Fowler 52-470-006 SteelTelescopic Gage with Double Action Rigid Handle


The Fowler gauge features a rigid rod, which is helpful for grip and comes with vinyl casing for storage.

6 Pc Precision Telescoping Gage Set 5/16″ – 6″ Range T-Bore Hole Gauges w/ Pouch


They get the job done, they’re no Starrett but you already knew that! As long as you check your work you will be good to go. Keep them safe in the included pouch!

Anytime Tools Bore Gauge 6 pc 5/16″-6″ Premium Telescopic High Precision T-Gage Set w/Hard Shell Case


Recommended if your project allows you to measure in fractions of an inch, not thousandths.


Beyond telescopic gauges, there are other tools that can help measure the diameter of a hole. If you’re interested in hearing what our members think about the topic and specifically telescoping gauges, join the conversation here.

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  • Norton says:

    I use them for bearing fit almost daily,never have I had a misfit using them. I find them easier to use than internal micrometers.

  • Bill3 says:

    Thank you for your presentation. Nice information. I would like to call attention to the use of the term snap gauges. Some person wrongfully called them that and the term has erroneously stuck. Please do not think these are snap gauges as that tool is completely different and it performs an entirely different function. No one would NEVER find telescoping gauges in a catalog under snap gauges or vice-versa. Please call it by its correct name and demonstrate one knows what they are talking about.

  • TGTool says:

    Years back I ran a job jig grinding steel rings that were intended as a shrink fit for a carbide slug something over an inch in diameter and we measured size with telescoping gauges. Inaccurate measurement meant that either the carbide wasn’t retained or the rings split on cooling.

    The foreman who taught me the nuances of that job had two rules for using them. 1. Wiggle the gauge though the diameter ONLY ONCE. Sliding back through again negated the measurement. 2. Remember the feel of the resistance as the gauge slid through and try to duplicate that feel as you tease the gauge back and forth between the mic anvils. We could establish size to under a thousandth.

  • Chip Burns says:

    A well written article as a place to start. There are two breeds of these animals. One has only one movable plunger, while the other has movable plungers on both ends. The second type will allow centering the handle closer to the bore centerline, and create a smoother motion past the true diameter. Getting the gauge handle as close to the bore center, and tilted only enough to place pressure on the plungers is the time to tighten the clamp screw. This bit of pre-location will allow the gauge to float to the true vertical center of the bore as the handle is pulled smoothly down. I have achieved better accuracy with a one pass movement, rather than back and forth. I was taught that you measure and write down the dimension. Then repeat to see if the same number keeps coming back. This is a game of consistency.

  • Rick says:

    Back in the eighties we were making some very tight tolerance parts for Robinson Helicopters. I measured the bore as being slightly over size and out of spec. The shop flunky who made them convinced the boss that we needed much, MUCH better measuring tools so he went out and spent over $800 on a set of dial bore gauges. And got exactly the same reading I got with a telescoping gage and my Alina micrometer. The flunky got fired for scrapping the parts and I laughed all the way home that night…

  • Eabrahim says:

    Veriy good

  • Rods says:

    Using a set of Starrett telescopic gages now for 2 1/2 years and they work great. It no problem being able to achieve repeatable measurements. I always make sure to drop the smaller pin side before I remove them to measure.

  • Norton says:

    nice,that is the way i do it 1-400 times a day

  • GH says:

    I routinely check inside diameters with my telescoping gauges with success. It should be mentioned that a “one pass” measuring technique will lead to failure, with any inside measuring tool it is good practice to check holes at several depths and at varying angles to the initial pass, this helps find out of round and tapered holes that would be missed with only one pass.

  • Don says:

    I prefer the type with just one expansion leg and I always drag the tool through the bore rather than pushing. I find this technique provides accurate and repeatable readings

  • Will B says:

    This article is spot on and pleasingly opens discussion for further knowledge sharing. My experience with telescoping gauges has taught me to always select a brand that has close tolerance of the gauge plungers to their tee-handle bores. Errors or difficulty in obtaining a measurement can result because of a sloppy gauge. Also be mindful that the telescopic gauge must have a concentric-ground profile on the domed end of the plungers. Again, errors can occur at the measuring of the bore and upon transferring that dimension to your micrometer because of a poor dome profile. And yet another check to perform before using the gauge is to check for “plunger-ramping”. This occurs when the gauge locking function is poorly designed. Look for variance of feel, when measuring the bore, between when the lock is free and when it is active. A good gauge will not change dimension measurement in either the free or locked position, while a poorly designed gauge-lock will cause the plungers to force outward or inward, giving a false reading of the bore size. In summary, you get what you pay for.

  • jondee says:

    In my experience these things can be pretty accurate. There is another way to use them. Take the measurement from the anvils of a micrometer to the work. The mike has hard and very smooth faces and a tenth of a thou can be felt without much trouble. Set the gauge to a Go or Nogo value and then try it against the work. Two gauges make go/nogo assessing faster. They are definitely things you use carefully when measuring. I also have a set of Starrett wedge gauges and these things are beauties in situations where you can get a mike on to the gauges when set in a hole.

  • Geof says:

    good article, i have used them for many years, worked in hydraulics, mold building, building racing engines, and currently an electric motor shop where bearing fits are only .0004 tolerance! it is a “feel” for the accurate use and repeatability! You always check at least 2 directions and repeat at least 2 times to ensure good sizing. I always release the gauge off center so they don’t snap open, this avoids possible dents in the bore and access wear on the gauge!

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