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A thread gage is a device that is used to check the acceptance of an internal or external threaded part. Each gage type is used to monitor the tolerance, strength, reliability and durability of each component to ensure overall quality. With a handful of different thread gage types, it can be hard to determine which is used for specific measuring. This comprehensive guide will help walk you through the application and usage of each specific gage.


When inspecting a threaded part, there are a handful of features that are involved – major diameter, minor diameter, pitch diameter, pitch, threads per inch (TPI), GO and NO-GO gages. Each of these terms are essential in the inspection process and need to be measured precisely. It is very important to understand what exactly these terms mean and how they are particularly related to the acceptance of a both internal and external threaded parts. Below you will find a brief description of each and how it pertains to the threaded part.

known as diameter of the imaginary cylinder which bounds the crest of an external thread or the root of an internal thread.
known as the diameter of an imaginary cylinder which bounds the root of an external thread or the crest of an internal thread.

  • Straight thread – the pitch diameter is known as the diameter of the pitch cylinder.
  • Perfect thread – the pitch diameter is known for where the widths of the thread ridge and the thread grooves are equal.
  • Thread gages -pitch diameters marked represent the minimum and maximum pitch of internal and external threads.

the distance between points on adjacent thread forms in the axial plane and on the same side of the axis
a count of the number of threads per inch measured along the length of a threaded part.
These two terms are used to describe if a part is usable or unusable. As stated previously, if a GO gage can correctly and smoothing fit, then the part is considered usable. If the NO-GO gage can successful turn more than two times, then the part is deemed defective


This type of thread gage is used to check the acceptance of an internal threaded part, otherwise referred to as a nut. For smaller threaded parts, the thread plug gage will consist of two ends, one end with a GO gage and another with a NO-GO gage. Larger threaded parts will usually require a gage with two separate pieces. This particular gage is designed to check the correctness of pitch diameter. In order for a threaded part to be deemed accepted, the GO gage should pass through the entire nut without using too much force. The NO-GO gage should not go beyond 2 turns.


There are two types of thread ring gages, solid and adjustable. Both gages are designed to check the correctness of thread, including thread depth and pitch diameter.

A solid thread ring gage is used to check the acceptance of an external threaded part, otherwise known as a screw. Similar to the thread plug gage, a solid ring gage will consist of a GO and NO-GO gage, however, the rings are normally separate pieces. For acceptance of a part, the GO gage must pass through the entire length of the screw without much force – particularly looking for smooth and easy transition. The NO-GO gage should not go beyond 2 turns.

An adjustable thread ring gage is more complex than the solid thread ring gage. Although similar in shape, an adjustable thread gage consists of a split with an adjusting and locking screw facility. This facility is used to adjust the right gage sizes. These gages require setting plugs in order to correctly set the size. For accuracy, two setting plugs are used, one for the GO side and one for the NO-GO side.


Similarly to a thread ring gage, a thread caliper gage is used to check an external threaded part. Although these gages are not as accurate as a thread ring gage, they are considerably faster. A thread caliper gage is designed with a roller type GO element, which simply slides over the threaded part using gravity. These gages require setting plugs in order to correctly set the inspecting size. Acceptance is deemed when the GO roller smoothly passes through the gage using gravity alone.


A thread pitch gage is not a typical inspection device. It does not decide acceptance of a threaded part. This type of gage is used for visual checking to identify a threaded parts pitch. It will usually come as a set of thin plates with teeth precisely machined to a given thread pitch (threads per inch/TPI). The plates must closely match up with the threaded part. When all the teeth match up properly with the part being measured, with absolutely no gaps, the pitch size engraved in the plate becomes readable. Notably, a thread pitch gage is not completely foolproof and unless the inspector is an expert, there is a possibility of making an incorrect determination of a pitch/thread type.


When it comes to caring for thread gages, three main factors come into play. From start to finish, thread gages must be properly used, maintained and stored. Each one of these three key factors contains multiple procedures that are essential in ensuring accurate test results and a longer life span of instrumentation.

  • Properly training any laboratory personnel involved with the usage of thread gages will ensure gages are being treated and used correctly.
  • Thread gages should never be forced into or onto any dimension being checked. Gages should be turned or pushed slowly and gently. Forcing a thread gage will result in not only faulty gaging, but also damage to both the gage and the part.
  • When using a thread gage, it is also important to remember to avoid touching gage surfaces. Oils found on the surface of our skin could lead to rust.
  • The temperature of the thread gage and the part should always be the same temperature. The ideal temperature for both parts is 68 degrees Fahrenheit. This helps effectively eliminate thermal expansion and areas that can be caused by it.
  • After each use, thread gages should be cleaned with a lubricant and then recoated with a thin layer of rust preventative. An alternative to this procedure is dipping a thread gage in an easy to peel oiled-based wax coating.
  • Frequent thread gage inventory checks should be down to look for visual signs of nicks, dents and scratches on the gages.
  • During inventory and before every use, thread gage recalibration dates should be checked to ensure the date is current.
  • Thread gages should be inspected and recalibrated periodically to ensure accuracy of tests and measurement. Inspection and recalibration frequency should be based of the amount of usage, the part and gage material, the tolerance and the quality procedures performed.
  • After gages have been properly maintenance, they should be stored by specific type in the correct plastic storage box, container or cabinet.
  • Thread gages should be protected from exposure to excessive heat, humidity, moisture and corrosive chemicals.

When these thread gage care procedures are performed properly and precisely, gages should be expected to have a longer life span. Gage life will also be determined by the frequency of usage and the material the gage is made out of. Notably, when investing in a thread gage, take into consideration the gage steel. It is important to look for material with extreme stability and a high wear resistance. The better the quality of the material, the longer the gage life expectancy.


Calibration of a thread gage defines the accuracy and quality of an internal or external measurement. Overtime, accuracy of a thread gage can start to drift due to excessive use of the instrument. By calibrating a thread gage frequently, measurement uncertainty minimizes which, in turn, ensures result accuracy. Accuracy of results is extremely important when it comes to use of equipment because it allows users and businesses to feel confident, reduces costs from manufacturing errors and keeps processes safe.


There a many deciding factors on when a thread gage should be calibrated. Calibration is required for the following:

  • Testing of a new thread gage
  • Testing a thread gage after it has been repaired or modified
  • Testing after specific usage of a thread gage
  • Prior to and after critical measurements of internal and external parts
  • When there is measurement uncertainty
  • After events where a thread gage has been dropped or misused
  • If a thread gage is exposed to adverse conditions

Thread gages should also be recalibrated periodically based on an interval, either recommended by the manufacturer or determined by a trend history analysis. When using an interval recommended by the manufacturer of a thread gage, it is important to remember the interval may decrease due to usage factors.

  • If a thread gage is not calibrated, it will not give accurate measurements. This leads to all of the points bulleted below.
  • When a thread gage is calibrated with a certificate, you can prove that the gage is well maintained and regularly serviced. Without a certificate of calibration, the reliability of the measurements is low. By performing calibration on a thread gage, you are ensuring the gage is performing within accepted standards and is achieving quality assurance.
  • A handful of safety risks can come from uncalibrated thread gages. Since thread gages are used to measure conjoining parts, such as nuts and bolts, it is crucial that the gage can measure these internal and external parts thoroughly. If not measured correctly, the part being measured could become faulty causing danger to those working on it or those using the equipment.
  • If the quality of your thread gage isn’t good or if the service is inferior, the customers will be unhappy. If a part becomes faulty due to your uncalibrated and inaccurate thread gage, the customers won’t hesitate to take their business elsewhere.


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