What is a bore gage?
A bore gage is a tool used to measure the inside of a cylinder or hole, which is also known as a bore.
The base of the tool is inserted into a bore. Then, anvils extend to perfectly match the shape of the space, allowing the user to read the diameter.
There are many types of bore gages with a range of features and strengths for different purposes. We’ll break down each model and how to use it, so you have all the details before you buy.
Bore gage applications
Many industries use bore gages, including automotive, manufacturing, and inspection.
Mechanics and machinists use bore gages to measure holes in engine blocks. Inspectors use them to measure inside injection moldings or the wear of extruder barrels. They’re also used to check the wearing of parts and to determine when the parts need to be replaced.
Types of bore gages (and how to use them)
There are a few main types of bore gages with varying designs. One of the more common styles has two anvils and needs to be calibrated with gage blocks.
Internal micrometers, or tri mics, have three anvils and are calibrated with setting rings.
A plug gage has plugs on either end of slightly different sizes. If the larger plug fits inside a bore, the hole isn’t sized correctly.
Here are more specific variations and how to use them.
Telescopic bore gage
A telescopic bore gage, also referred to as a snap gage or telescoping gage, has a T-shaped design. It has two telescopic rods with an internal spring and a handle with a locking screw. Some telescopic gages have one plunging rod, and others have two.
To use a telescopic bore gage:
- Rotate the locking screw counterclockwise to collapse the measuring heads
- Insert the gage into the bore you’re measuring and lock it in place.
- Rock the handle to compress the anvils.
Now you can use another tool like a micrometer or caliper to measure the fixed gage to get the diameter of the bore.
Small-hole gage
As the name suggests, a small-hole gage is used to measure smaller-sized bores. Small-hole gages come with either split-ball or full-ball contacts. Split-ball has more spring, which makes the full-ball a bit easier to use.
Once you place the gage into the bore, adjust it until you make contact with the edges of the bore. Then you’re ready to measure the gage with a micrometer or caliper.
Dial bore gage
The dial bore gage is slightly different from telescopic and small-hole gages. It has three anvils that extend from the body of the gage.
To use a dial bore gage:
- Insert the gage in the bore.
- Rotate the knob to extend the anvils.
- Once the anvils make contact with the edges of the bore, you can read the measurement directly on the tool. No micrometer needed!
- Remember that a dial bore gage must be calibrated before each use.
Digital bore gage
A digital bore gage is the most advanced gage, yet easy to use. They vary in design and have either two or three anvils that extend to fit the size of a bore.
Digital bore gages may use a cable or wireless technology to make a direct measurement reading. Some even come with attachments to measure different sized holes.
FREQUENTLY ASKED QUESTIONS
- What is a bore gage used for?
A bore gage is used to measure the internal diameter of a bore or hole. Bore gages have numerous applications across industries, including:
- Measuring the interior of injection moldings
- Checking the wear of cylinder heads and extruder barrels
- Measuring holes in an engine block to ensure pistons fit tightly
- Determining when various parts need to be replaced (by measuring wear)
- What are the general styles of bore gages?
There are three broader types of bore gages. The first is designed with two anvils and requires calibration with gage blocks.
The second has three anvils and is calibrated with setting rings. This is called a tri mic, or internal micrometer.
The third (and simplest) type is a plug gage. It has plugs of different sizes on either end, and the larger plug won’t fit a correctly-sized bore.
- How do you use a bore gage?
There are many types of bore gages, and each works slightly differently. But the general usage is the same. To use any bore gage:
- insert the gage into the bore and extend the anvils (some will have two anvils, others have three) until they touch the edges of the hole.
- Once you feel a slight pressure, you’ve made contact.
- If you are using a dial or digital bore gage, you can make a measurement reading at this point.
- When using other bore gages, such as a telescopic or small-hole gage, use a caliper or micrometer to measure the fixed gage.
GLOSSARY TERMS
- Bore gage
A bore gage is a precision measurement tool used to determine the internal diameter, radius, or circumference of a hole, also known as a bore. Bore gages are used in multiple fields of mechanics and come in a variety of types, used for different purposes when considering the job at hand and the preference of the user. The different varieties of bore gage include: telescopic bore gage, small hole bore gage, beam bore gage, and dial bore gage. Telescopic, small hole, and beam bore gages all involve a transfer process to get a final measurement, whereas a dial bore gage measures a bore directly. Small hole bore gages come in the full-ball or half-ball variety. Dial bore gages also come in electronic or wireless versions, which can be used to get an even higher level of precision and accuracy.
- Anvil
The longer, straight edge of a combination square handle used to measure 90-degree angles.
- Telescopic rods
the parts of a telescoping bore gage used to measure the internal diameter of a bore. Telescopic rods have an internal spring and a handle with a locking screw. Telescopic bore gages may have one or two plunging rods.