All objects have some sort of connective facet holding them together. In the case of vehicles, machinery, medical devices or industrial valves, that connective element often consists of various nuts, bolts and component parts that must be assembled.

What makes these items function the way they are supposed to is their “thread.” Design engineers pick specific types of threads that will best suit the function of the finished product. Specific threads are chosen to pull, push, regulate, fasten, mount or permanently hold. A thread is a superior form of fastening that offers a much stronger hold when stressed near limits.

Thread Gage Basics

In uses where each nut, bolt or part component has to support equal stress or weight, it is particularly important that the load be borne over the entire object. The time to figure out whether that will happen is before the object is tested, and this action is best accomplished by standardized sizes and strengths for all connecting hardware that can be verified.

Most threads have dimensions that correspond to a specific standard. Industry adheres to the standards by American National Standards Institute (ANSI), Japanese Institute of Standards (JIS), British Standards Institute (BS) and the German Institute of Standardization (DIN).

What happens if one cannot verify those attributes? The worst-case scenario is non-assembly or scrap. Part failure and breakage also could occur.

Consider a car. Exactly how confident would you be driving a vehicle that was held together with a variety of nonconforming nuts, screws and bolts? Thread gages, in short, allow mass-produced threaded components to be gaged for uniform tolerance, which means strength, reliability and overall durability.

Depending on the item to be measured, there are several fixed limit gages that assist in the measurement process. Gages must be used to measure both the internal and external threads in order to comply with the class of fit as specified on the component drawing.

Fixed limit thread gages are the most popular measuring device on the market due to their reliability, cost effectiveness and ease-of-use. Machine operators and inspectors use them throughout assembly and at final inspection for quality assurance.

Working and Master Gages

Working gages allow an operator to check and verify the correct size and pitch diameter of the external and internal thread. These gages are intended to be accurate to millionths of an inch, essential for such sensitive uses on aircraft, spaceships and equipment that take on big stress loads, and they must be manufactured to gagemakers’ tolerances that are published by ANSI.

Thread plug gages, which are manufactured to ANSI standards for length and dimensions, include:

Go Thread Plug. Common to all classes of thread, checks pitch, lead and angles, but not the minor diameter. Identified by nominal size, threads per inch, series and go pitch diameter, for example; 250-20 (¼-20)UNC GO P.D. 0.2175, a thread plug gage should enter the full threaded length of the product internal thread completely and should never be forced with a wrench.

No-go Thread Plug. Sometimes called HI and is made to the maximum pitch diameter of the internal thread. The major diameter of a no-go is flattened or truncated, allowing the flanks of the gage to check the maximum geometric pitch diameter.

The no-go should never enter the product thread or be forced. Some inspectors will allow the no-go to enter the part for three threads and then stop. It also is important to know that the no-go working thread plug is shorter in length than the go.

Thread Ring Gages

Thread ring gages need to be calibrated with another measuring gage called a master plug or setting plug that checks and re-checks the calibration of the original gage.

Go thread rings check all elements of the external thread, except the major diameter and should be set to its corresponding master setting plug set at “W” tolerance. The no-go thread ring checks the part at the minimum material pitch diameter. The part should not enter the no-go ring and should not be forced. Some inspectors use the rule that the no-go ring can go on the part for three threads and then stop. If doubts about the part’s low limit acceptability occur, then gaging by variables and/or direct measurement of the part should be considered.

Master gages are often referred to as setting plugs, master or reference gages and wear check plugs. They are made to closer tolerances than working gages. Truncated setting plugs are used to set or calibrate adjustable style rings.

Master gage blocks, master or setting discs and setting rings are types of master gages used to calibrate or set micrometers, comparators or other gaging systems.

Common Terms

Pitch Diameter. The pitch diameter on a straight thread is the diameter of the pitch cylinder. In a theoretically perfect thread, the pitch line, parallel to the part axis, is located where the widths of the thread ridge and the thread groove are equal. Pitch diameters marked on thread gages represent the maximum and minimum pitch diameters of the product external or internal threads.

Go and No-Go. These terms describe gages that are used to determine whether a part is usable or unusable. A “go” gage would correctly determine whether a part can be used, and a “no-go” would successfully determine a defective part.

Class X Tolerance. All working gages are made to these gagemakers’ tolerances. Example: ¼ -20 UNC -2B Go pitch diameter 0.2275+0.0003 (+0.0003 is the class X tolerance). A high limit for this gage is 0.2278.

Class W Tolerance. All setting plugs are made to these gagemakers’ tolerances. Example: ¼-20 UNC -2A Go pitch diameter 0.2164 - 0.0001.



Gages are always identified to their corresponding nominal major diameter and threads per inch:



Examples - Series
5/16-18 UNC - Unified National Coarse
5/16-24 UNF - Unified National Fine
5/16-32 UNEF - Unified National Extra Fine
5/16-20 UN - Unified National
5/16-36 UNS - Unified National Special


In order to increase life cycle of a thread gage, make sure to always use the correct class of fit and tolerance disposition. Also clean the gage after each use by applying oil or another suitable form of anti-rust formula to prevent damage to the external diameter.