Do you need to measure the diameter of a hole? Before you reach for your trusty caliper or start to fumble around swapping micrometer anvils, it’s essential to know which tool is most suited to your specific application.

In this article, we look at the main types of bore gauges available. Assessing their functions and suitability for different applications.

Before we get into the different types of bore gauges, let’s first recap what a bore gauge is:

1. What is a Bore Gauge?

Bore gauges are measuring instruments specifically used to measure the internal diameter of spherical objects such as a hole, pipe or cylinder, a bore.

2. How Does a Bore Gauge Work?

The different types of bore gauges are discussed later in the article. Although they may differ in measuring technique and format, in basic principle, bore gauges have contact measuring points that enter the bore before expanding out to make contact with the internal surfaces of the hole. Once the contact points are touching the internal walls of the item, the gauge will display the measurement of the bore diameter.

3. Measuring Head Points

All types of bore gauges use either a two or three-point measuring system. Meaning they either have two or three points of contact with the wall of the bore to establish the diameter measurement.

Let’s discuss each system and look at the advantages and disadvantages of each.

2 Point Bore Gauges

Two contact points on the measuring head expand out to meet the internal surfaces. The two-point system works better if the bore has ovality to it.

3 Point Bore Gauges

Having three contact points offers more accurate measurements when the bore is round or close to being round. However, as demonstrated in the diagrams below, three points will not provide the most accurate measurement when the bore has ovality as it will fail to measure the maximum diameter of the bore. 

In reality, no hole is exactly round or oval. The three-point system will offer the most accurate readings on holes with deformities. Except where the bore has a clear ovality when a two-point system will be more suited.

4. Setting Rings

Setting rings are a calibration tool used to set and check the bore gauge against a known dimension.

The rings allow you to check the repeatability and accuracy of the gauge, as well as setting the gauge at a specific, accurate measurement that will be used as a comparison measurement for your application.

We’ve looked at the operation of bore gauges and the measuring systems they use, now let’s look at the different types of bore gauges and their specific applications.

5. Types of Bore Gauge

Telescopic Gauge

The simplest type of bore gauge, a telescopic gauge is an indirect measuring instrument used in conjunction with a micrometer to achieve accurate measurements of bores. Available in a range of sizes, the arms of the t-shaped instrument are expanded and locked to fit into the diameter of a hole before using a micrometer to measure the distance across the arms.

Cylinder Bore Gauge

The cylinder bore gauge has a long shaft with a dial indicator on the top and a measuring head on the bottom. It can be used to quickly and accurately measure cylindrical bores through its two-point measuring system. While the movement on this type of gauge is quite small, they’re normally supplied with a number of different sized interchangeable measuring contacts which allows for a wide measuring range.

A Cylinder Bore Gauge works on a similar principle to the telescopic bore gauge. The anvils of the gauge are held inside the bore. They are then brought into contact with the surfaces of the hole to establish the diameter. The measurement is then displayed either on a analogue dial or a digital screen. This means a micrometer isn’t needed to quantify the reading. However, using either a micrometer, or ideally a setting ring to calibrate the instrument before use is required.

Standard Rotary Bore Gauge

Standard rotary bore gauges offer ease of use and accuracy. Commonly this type of gauge uses a 2 point system below 6mm, and 3 point above 6mm. They’re often available in a wide range of sizes and also in sets incorporating different sized heads to cover a large range. Extension rods are also often available which increase the measuring depth of the gauge.

Pistol Grip Bore Gauge

A popular choice for a wide range of applications, pistol grip bore gauges offer fast, one-hand operation. They perform well in repeatability (repeat measurements taken by the same user) and reproducibility (repeat measurements taken by different users).

Internal Micrometer

An internal or inside micrometer has protruding arms meaning the jaws can be placed into a bore and then moved into contact with the internal surfaces to take the measurement. Of course, this is a two-point measuring system that we know is limited. Another limitation may be the length of the jaws if working on a deep bore

Standard Stick Micrometer

A two point system, the micrometer is placed inside the bore before the points expand out to meet the wall of the bore. The user can then turn the micrometer around the circumference of the hole to find inconsistencies and measure the maximum radius. Technique and skill are important when using this type of gauge.

Find out more about the recommended Mitutoyo Pencil Micrometers on their website.

6. What to look for in a Bore Gauge

Now you have an idea of the type of bore gauge suitable for your application. We’ll look at the general specifications of bore gauges to ensure your choice provides the operational accuracy and reliability you need to achieve your desired results.

Range

The range of the instrument states the size of bore it is suitable to measure. You should have an idea of the size of the bore you need to measure, so you will know if the contact points will reach the surface of the bore! If you’re measuring different-sized holes, you may need a bore gauge set with several gauges or interchangeable heads to cover the full range of bores. 

Measuring Depth

If you need to reach inside the bore at depth to take the internal measurement, the measuring depth of the gauge will state how far into the bore it will go. Extension rods which increase the measuring depth are available for most rotary and pistol grip gauges.  

Resolution

The resolution is the smallest unit of measurement the instrument is capable of registering. Expect a resolution between 0.01-0.001mm on most bore gauges. Again, you’ll have an idea of how accurate your measurements need to be.

Accuracy

Shown with a ± symbol, the accuracy of a bore gauge details the margin of error the instrument will produce readings within. The reading may be above or below the true measurement of the item within the stated accuracy margin.

Repeatability

Repeatability is the deviation expected when the same person repeatedly uses the bore gauge to measure the same bore under repeated conditions. A consideration for many applications, particularly when the tool will be used to quality check a large number of products. 

Reproducibility

At times confused with repeatability. Reproducibility states how accurately the bore gauge will produce consistent readings of the same application taken by different users under the same or different conditions. 

Type of contact points/measuring heads

Specially shaped contact points are available for specific applications when needed. The material of the contact point is also a consideration. Carbide or carbide tipped points are strong, long-lasting, and generally the most common.

IP Rating

Exposure to dust and liquid is often a consideration for handheld metrology equipment in outdoor or workshop environments. The IP rating of an electrical instrument states its level of protection against dust and liquid ingress.

Read more on the specifics of IP rating in our article: What are IP Ratings? 

See the article here…

Now you have a clear idea of the type of bore gauge you need. Let’s consider specific brands and models.

7. The Best Bore Gauge Manufacturers

Bowers Bore Gauge

Bowers is the first brand name often associated with bore gauges, and for good reason. Their product range is impressive, as is the quality of their instruments. As the worlds leading bore gauge manufacturer, they are continuously looking to push the capabilities of their equipment through design innovation and new technologies. The pistol grip bore gauge is a good example of this, the model was first introduced by Bowers and has quickly become the gold standard in handheld bore gauges.

Other impressive features of Bowers bore gauges include:

• Customisable ratchet force – a hugely advantageous feature of Bowers bore gauges is the settable ratchet force
• Sylvac wireless data output – Sylvac bluetooth/wireless connectivity and corresponding features are second to none

Shop our range of Bowers bore gauges here

Mitutoyo

A company with a lofty, and well deserved reputation in the metrology world, you can read more about Mitutoyo and their range of micrometers here. Their range of internal gauges include Telescopic gauges, standard rotary bore gauges, snap-open bore gauges (Similar to a pistol grip design in that they’re lever operated), cylinder bore gauges, stick micrometers and internal micrometers.

Shop our range of Mitutoyo bore gauges here

Moore & Wright

Moore & Wright offer good quality metrology instruments at affordable prices and their internal micrometers, cylinder bore gauges and telescopic gauges are no exception.

Shop our range of Moore & Wright bore gauges here

DML

Our own range of Bore Gauges offers premium quality at a fraction of the price of big brand names. We stock both digital & mechanical gauges which includes sizes from 2mm to 300mm.

Shop our range of DML bore gauges here

Conclusion

There are many factors when considering which type, brand, and model of Bore Gauge is most suitable for your application. If you need a tool with a wide measuring range, for an application that’s difficult to get to, and needs to be measured quickly, and accurately (while putting the kettle on with the other hand!), then a Bowers pistol grip set may be your best option. However, if you are simply looking to measure one bore accurately, a simple, and much cheaper cylinder dial bore gauge may well suffice.

We hope this guide has given you the knowledge to confidently select the type of bore gauge you need. However, please do get in touch if you have any further questions.

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