Checking internal threads with a thread plug gauge: How to do it correctly
An internal thread can look cleanly cut and still be outside the permissible tolerance. The thread flanks are visible, the surface looks neat and a screw may even start to engage. However, only an inspection with the correct precision thread plug gauge shows whether the thread actually fits.
“A thread inspection is not about whether a thread looks good. What matters is whether it lies within the permissible tolerance.”
Precision thread plug gauge, GO plug gauge or NO-GO plug gauge: What do these terms mean?
The general term for these inspection tools is precision thread plug gauge. Depending on the thread size and thread type, the design may differ.
For smaller dimensions, a thread limit plug gauge is often used, with the GO side and NO-GO side combined in one inspection tool. This is common for metric coarse threads up to M36 and metric fine threads up to M40.
For larger dimensions, the GO and NO-GO inspections are carried out separately. For metric coarse threads from M39 and metric fine threads from M40, separate inspection tools are therefore used:
- GO plug gauge: for the GO inspection
- NO-GO plug gauge: for the NO-GO inspection
For other thread types, such as inch threads, the arrangement is similar. The decisive point is always that the plug gauge matches the thread type, thread size and tolerance class.
What does a GO and NO-GO plug gauge inspect?
A GO and NO-GO plug gauge checks internal threads against clearly defined limit dimensions rather than by feel. Whether both sides are combined in one thread limit plug gauge or designed as separate GO and NO-GO plug gauges depends on the dimension and thread type.
The inspection principle remains the same:
- GO side: It must screw in easily and completely. The thread is then generally usable.
- NO-GO side: It must not screw in completely. Otherwise, the thread is too large and lies outside the permissible tolerance.
This quickly shows whether an internal thread is acceptable or not. The plug gauge does not measure every individual thread value separately; instead, it answers the key practical question:
Is the internal thread within the permissible limits?
This is precisely why the inspection is so practical in workshops, quality assurance and series production. It is quick, unambiguous and can be carried out directly on the component.
GO side and NO-GO side: What is the difference?
Thread inspection always concerns two limits. The thread must not be too tight, but it must not be too large either. This is exactly what the two inspection sides, or the two separate plug gauges, are designed for.
| Feature | GO plug gauge | NO-GO plug gauge |
|---|---|---|
| Purpose | checks whether the internal thread is usable | checks whether the internal thread is too large |
| Use | must screw in easily and completely | must not screw in completely |
| Result | thread is not too tight | thread is not too wide |
| Typical problem | GO side jams or does not go in deeply enough | NO-GO side screws in too far |
| Most important point | do not apply force | do not “keep turning by feel” |
In brief: The GO side must go. The NO-GO side must not go. Only when both conditions are met is the internal thread within the permissible range.
How to inspect an internal thread correctly
The thread should be clean before inspection. Chips, dirt, oil residues or burrs can distort the result. It is therefore worth taking a quick look inside the component, especially immediately after thread cutting or thread forming.
The GO side is used first. The GO plug gauge is aligned straight and screwed into the internal thread by hand. The following rules apply:
- align it straight
- inspect by hand
- do not tilt it
- do not apply pressure
- do not use a tool
The GO side must screw in easily and completely. The NO-GO side is then used. It is also aligned straight and checked only by hand. The NO-GO plug gauge must not screw in completely.
For metric internal threads, a common practical rule is that the NO-GO side must not screw in by more than approximately two turns. However, the applicable standard, tolerance class and inspection specification are always decisive.
What does it mean if the thread fails the inspection?
If the thread fails the inspection, the respective side already provides a good indication of the direction of the problem:
- GO plug gauge does not enter cleanly: The thread is usually too tight, not fully produced or obstructed.
- NO-GO plug gauge enters too far: The thread is usually too large or outside the permissible tolerance.
The causes may lie at different points in the process:
| Possible cause | What may be behind it |
|---|---|
| Core hole is incorrect | The core hole was too small or too large. As a result, the internal thread later becomes too tight or too wide. |
| Thread is not deep enough | The GO side cannot screw in completely because the usable thread depth is insufficient. |
| Tool is worn or unsuitable | The tap or thread former no longer works cleanly or is not suitable for the application. |
| Tool cuts or forms oversize | The NO-GO side engages too far because the thread lies outside the tolerance. |
| Chips, burrs or damage | The thread is obstructed or runs unevenly. |
| Plug gauge applied at an angle | The inspection result is distorted even though the thread may actually be acceptable. |
| Incorrect thread data | Thread size, pitch or tolerance class do not match the gauge being used. |
| Inspection tool is not in proper condition | The plug gauge is worn, damaged or no longer reliably calibrated. |
For this reason, an unusual inspection result should not immediately be attributed only to the component. Sometimes the fault lies in the thread itself. In other cases, it lies in the preparation, the tool, the wrong gauge or the way the inspection tool is handled.
Why the correct tolerance is decisive
For internal threads, the 6H tolerance is very common, for example. This designation is part of the thread specification and must match the gauge being used.
An M6 x 1 6H plug gauge checks an M6 metric internal thread with a pitch of 1.0 mm and a 6H tolerance. This is why four points should always be clear before inspection:
- thread size
- pitch
- thread type
- tolerance class
Only when these details are correct can the appropriate GO and NO-GO plug gauge be selected. Otherwise, an acceptable thread may incorrectly be classified as rejected. Or a non-conforming thread may accidentally be approved.
Typical mistakes during thread inspection
During thread inspection, the result depends not only on the thread itself, but also on how the plug gauge is handled. Three points are particularly critical:
- too much force
- the wrong gauge
- a damaged or unchecked inspection tool
If a plug gauge enters the thread only with pressure, pliers or another tool, the inspection has not been passed. The thread is no longer being inspected; the result is being distorted.
Using the wrong gauge also quickly leads to incorrect decisions. The plug gauge must always match the thread size, pitch and tolerance.
The condition of the inspection tool is equally important. Plug gauges are precision tools. They should be stored cleanly, protected against impacts and inspected or calibrated regularly depending on their use.
In brief: The plug gauge must be correct, guided properly and itself be in good condition. Only then is the inspection result reliable.
Inspect threads reliably with VÖLKEL
Thread inspection is an integral part of producing a clean thread. A thread is not automatically acceptable simply because it has been cut or formed. Only the inspection shows whether it is actually within the required limits.
We offer suitable thread gauges for inspecting internal threads. These include precision thread plug gauges, thread limit plug gauges, GO plug gauges and NO-GO plug gauges for different thread types, sizes and tolerances.
Inspection directly after machining is particularly useful. This makes it possible to identify early whether the core hole, tool, cutting depth and thread quality are properly matched. Errors are detected not only during assembly, but directly where they occur.
The advantage is greater process reliability:
- reduce rework
- avoid faulty connections
- release components with confidence
- identify assembly problems earlier
- check thread quality directly on the component
This turns a cut or formed thread into an inspected connection.
Conclusion: A thread is only truly reliable once it has been inspected
An internal thread can look good and still lie outside the tolerance. This is why inspection with a GO and NO-GO plug gauge is so important.
Whether as a combined thread limit plug gauge or as separate GO and NO-GO plug gauges, the principle remains the same. The GO side must screw in easily and completely. The NO-GO side must not enter completely. This is what shows whether the thread lies within the permissible limits.
Anyone who inspects threads regularly detects errors earlier and provides greater reliability in workshops, quality assurance and series production. With the appropriate VÖLKEL thread gauges, internal threads can be checked reliably – directly after machining and before assembly.
FAQ about inspection with GO and NO-GO plug gauges
What is a precision thread plug gauge?
A precision thread plug gauge is an inspection tool for checking internal threads. Depending on the design, it may be a thread limit plug gauge with a GO and NO-GO side or a separate GO plug gauge or NO-GO plug gauge.
What is a GO and NO-GO plug gauge?
A GO and NO-GO plug gauge is an inspection tool for internal threads. The GO side checks usability, while the NO-GO side checks whether the thread is too large.
What does the GO side inspect?
The GO side shows whether the internal thread is not too tight and whether a matching screw can later engage correctly.
What does the NO-GO side inspect?
The NO-GO side shows whether the internal thread is too large. If it screws in too far, the thread lies outside the permissible tolerance.
May force be used during inspection?
No. A plug gauge is used by hand. If it enters the thread only with pressure, pliers or a tool, the inspection result is not valid.
Why is the tolerance class important?
The tolerance class defines the limits within which the thread must lie. The plug gauge must therefore always match the thread size, pitch and tolerance, for example M6 x 1 6H.
