Telescoping 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.
There are telescopic gauges with two plunging telescopic rods and others with only one. With the help of a micrometer, the telescopic rods measure the distance of the bore. The ends of the rod are the shape of a half circle (called the radius edge), which is responsible for getting accurate measurements. Gently twisting the locking screw clockwise secures the telescopic rod(s) into position. When twisted counterclockwise, the screw is released, and the telescopic rods can move freely again.
Developing a good feel for the tool requires a little bit of practice. Collapse both measuring heads by rotating the locking screw counterclockwise. 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 by too much because the rods are spring-loaded. Proceed to tilt the gauge so the rods are inclined just a touch above horizontal, then tighten the locking screw (clockwise). Whichever end you lifted first, force it downward (the opposite way), slowly 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. Finally, measure with a calibrated micrometer.
Due to the touchiness of telescopic gauges, different machinists have different techniques. Some metalworking community members argue telescoping gauges are unreliable in terms of repeatability. However, once you develop your feel for the tool, they are a good option. Repeatable results just take a bit of practice.
Here’s machinist and YouTuber Adam Booth with his non-traditional approach.
Starrett S579GZ Self Centering Telescoping Gauge With 2 Telescoping Arms Set
With two plunging telescopic rods, this set is best for measuring both to tenths and thousandths.
Mitutoyo 155-903, 5/16" to 6", 6-piece Telescoping Gage Set
Best for projects requiring long reach. The set has two plunging legs and is known to provide good feel and lock up.
Fowler 52-470-006 Steel Telescoping Gage with Double Action Rigid Handle
Features a rigid rod which is helpful for grip.
6 Pc Precision Telescoping Gage Set 5/16" - 6" Range T-Bore Hole Gauges w/ Pouch
They’re no Starrett, but you already knew that! Doublecheck your work and you will be good to go.
Anytime Tools Bore Gauge 6 pc 5/16"-6" Premium Telescopic High Precision T-Gage Set w/ Hard Shell Case
Best for projects that need to be measured in fractions of an inch, not thousandths.
There are other tools that can help measure the diameter of a hole. If you’re interested in hearing what Practical Machinist members think about bore measuring tools, join the conversation here.
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