Common Uses for Bore Gages

A bore gage is a tool you use to measure a bore. No, not the kind that makes you feel tired and dull and like you want to leave a party. We’re talking about the kind that is synonymous with a hole. These tools come in handy across several different industries. You’ll find them in the toolkits of automotive technicians, manufacturers, and machinists.


Bore gages go by many other names, too. Common terms include hole tests, cylinder tests, internal micrometers, hold bore gages, and bore mics. The number of different names might make bore gages seem more complicated than they are. Most of these tools are straightforward to use. When calibrated correctly, bore gages provide accurate reads and come in handy for many different circumstances.


What are bore gages?


When it comes to measuring a hole’s diameter, nothing does the job like a bore gage. Bore gages are precision tools designed specifically for this purpose. They are simple in design, although they do vary depending on the type.


Many bore gages are designed with two or three telescopic rods — also referred to as legs. These bore gages are known as telescoping gages. In a telescoping gage, one or more of the rods are built with an internal spring. These rods adjust to the dimension of the hole. The person performing the measurement then compares that measurement to a measuring tool like a micrometer.


Other bore gages may have a dial built into them. These tools — known as dial bore gages — will read the measurement directly.


To get further into the what’s and whys of bore gages, let’s take a closer look at the different types.

 telescoping bore gage_alliance calibration

Telescoping gages


A telescoping gage is a simple and affordable solution for measuring the diameter of holes. These devices come in sets with a range of sizes. Each tool is generally shaped like a “T” and has retracting telescopic rods that will shrink or extend to fit the diameter of the hole.


A person can generally get the hang of a telescoping gage upon first use. After selecting the right size tool, the head of the tool should be placed inside the hole. A locking screw at the bottom of the gage tightens to hold the telescopic rods in place. Then, the person can remove the gage, and measure the size of the head with a secondary measurement tool.


The need for another tool is one of the downsides to the telescoping gages. It’s not always convenient to grab a micrometer when you’re trying to get a hole’s measurement quickly. But, because these are so simple and low-cost, telescoping gages are still a popular choice.

 bore gage with set ring_alliance calibration

Dial bore gages


Even though a dial bore gage should be calibrated before each use, it’s one of the most popular types of bore gages. For one thing, dial bore gages are among the most accurate types of bore gages.


These devices are also some of the easiest to use of all bore gages. They are simple to position, and unlike some gages they require no secondary measurement. Instead, they include a dial that provides the measurement read out. This eliminates an entire step from the measuring process described in the telescoping gage.


People who need to measure bores frequently tend to go with a dial bore gage because of its many conveniences. Set up is simple, and the dial even tells you when the tool is situated perpendicularly to the bore axis. These work well on deep holes and are adjustable so that they can measure a range of diameter sizes.


Because of their straightforward design, the dial bore gage is also accessible to those who have never used a bore gage.

 digital bore gage_alliance calibration

Internal micrometers


Internal micrometers — also called inside micrometers — are extremely accurate in their measurements. They read the measurement directly, so if you have a set on hand and know how to use them, they are extremely reliable.


The trouble with internal micrometers is that they have a learning curve. There are two versions of these tools — a jawed micrometer and a tubular micrometer. Both go directly into the bore and expand to the diameter of the hole to provide the measurement. Although that sounds simple, it can be difficult to ensure that the device is perpendicular to the hole’s axis. If it’s at all off, the measurement will be inaccurate.

 3 point bore gage_ alliance calibration

Three-point bore micrometers


A three-point bore micrometer offers the best of all worlds — almost. Their drawback is their price. Because this type of bore gage is so accurate and so easy, they’re pricey. They seem especially expensive compared to the simple telescoping gage. However, if you’re in an industry where you need to measure the diameters of holes often and quickly, a three-point bore micrometer is worth it.


This device looks a little like a hot glue gun. You point the gage at the hole and the dial at the back of the tool provides the read out. The result is an accurate and quick read, even on deeper holes.


Common uses for bore gages


None of this information is helpful if you don’t know when to use your bore gages. So, what experience calls for one of these devices?


Well, pretty much any time that a pipe, a cylinder, or a hole need measured, a bore gage will come in handy. You’ll find needs for these tools in automotive, inspection, and manufacturing industries. For instance, an inspector might use a bore gage to check the interior dimensions of an injection molding to ensure quality control. In another instance, they might use one to measure the wear in a cylinder head or an extruder barrel.


Automotive mechanics find a number of uses for bore gages. In addition to measuring a cylinder’s wear, they use them to get an accurate read on the holes in an engine block. With an accurate measurement, they can ensure that the pistons will fit tight enough to avoid gas leaks.


Every industry that relies on bore gages should also know the importance of regular calibration. With multiple moving parts and internal mechanisms, bore gages should be calibrated often. Sticking to the manufacturer’s recommended schedule will ensure that they continue to give accurate measurements.


You might also want to read:

What is a Micrometer?

What is a Snap Gage?

What are Vernier Calipers?




Phil Wiseman

Phil Wiseman

Phil Wiseman is Chief Marketing Officer at Alliance Calibration. He earned a B.S. in Chemical Physics from Centre College. Phil is an ASQ Certified Quality Auditor and ASQ Certified Manager of Quality/Organizational Excellence.

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