Thread inspection: GO and NO-GO explained simply
The screw fits. But does that mean the thread is within tolerance? Not necessarily. The fact that a screw can be inserted without difficulty does not mean that the manufactured thread is within the specified tolerances. This is exactly where GO and NO-GO thread gauges come into play.
GO denotes the acceptance side and NO-GO the rejection side of a thread gauge. The GO side must screw in over the entire thread length being inspected. For metric ISO threads, however, the NO-GO side may engage by no more than two turns.
What may sound contradictory at first is an important distinction: NO-GO does not mean that the gauge must not engage at all. Brief engagement is entirely permissible.
However, the two-turn rule does not apply to every thread system. The most important differences at a glance:
| Thread system | Standard | Inspection criterion |
|---|---|---|
| Metric ISO thread (M/Mf) | DIN ISO 1502 | NO-GO: max. 2 turns |
| Parallel pipe thread (G) | DIN EN ISO 228-2 | NO-GO: max. 2 turns |
| Whitworth thread (BSW/BSF) | BS 919-2 | Ideally no NO-GO engagement; up to 2 turns permissible |
| American Unified thread (UNC/UNF) | ASME B1.2 | NO-GO: max. 3 complete turns |
| Pipe thread (R/Rp/Rc) | ISO 7-2 | Inspection with special limit gauges and reference faces |
| Tapered pipe thread (NPT) | ASME B1.20.1 | Special case: inspection with special limit gauges and MIN/MAX inspection steps |
Plug gauge or ring gauge: Which thread gauge do you need?
Whether you need a thread plug gauge or a thread ring gauge depends on whether you want to inspect an internal or external thread.
Thread plug gauges for internal threads
For internal threads, for example in nuts, housings or tapped holes, you use a thread plug gauge. Depending on the thread type and thread dimensions, the GO and NO-GO sides are either combined in a single limit plug gauge or available as separate GO and NO-GO plug gauges.
On a combined limit plug gauge, the two inspection sides are located at opposite ends of a handle. On VÖLKEL gauges, you can identify the rejection side by the red ring and the shorter gauging thread.
Thread ring gauges for external threads
External threads on screws, bolts or shafts are inspected with thread ring gauges. Unlike the combined limit plug gauge, GO and NO-GO are two separate inspection devices here. The GO ring gauge and the NO-GO ring gauge are screwed onto the external thread one after the other.
In both cases, it is essential that the gauge matches the thread size, pitch and tolerance class of the workpiece. For an M8 × 1.25 – 6H internal thread, for example, you need a thread gauge made to the corresponding specification.
You can find suitable inspection equipment from VÖLKEL under thread plug gauges for internal threads and thread ring gauges for external threads.
How should the GO side screw in?
The GO side, also known as the acceptance side, must screw in or screw on by hand over the entire thread length being inspected. Excessive force must not be required.
Position the gauge straight and turn it in evenly. If it can be screwed in completely, the GO inspection has been passed. For blind holes, the required usable thread depth is decisive, not the full depth of the tapping-size hole.
If the GO gauge stops prematurely, the inspection has initially not been passed. Possible causes include an internal thread that is too tight or an external thread that is too large, deviations in the thread geometry or damage. Chips or burrs can also affect the inspection.
How far may the NO-GO side screw in?
The NO-GO side, also known as the rejection side, checks the opposite tolerance limit. For metric ISO threads, it may screw in or screw on by hand by no more than two turns.
The rejection side does not necessarily have to reach two turns. If it stops after half a turn or one complete turn, that is generally permissible.
If, however, it can be screwed in by more than two turns, the inspection has not been passed. On an internal thread, this indicates that the pitch diameter is too large; on an external thread, that it is too small.
An important detail from DIN ISO 1502: The number of turns is determined while unscrewing the rejection gauge. Threads that are accessible from both sides are inspected from both ends. For very short threads with no more than three thread turns, the NO-GO gauge must also not pass completely through or over the thread.
Practical example: M8 × 1.25 – 6H internal thread
Suppose you want to inspect a manufactured M8 internal thread. The GO side screws in without difficulty to the required thread depth. You then use the NO-GO side, which stops after just one turn.
The result: Both the GO and NO-GO inspections have been passed. The thread therefore meets the corresponding inspection criteria of DIN ISO 1502.
What is actually checked during thread inspection?
A thread gauge checks more than just the thread diameter. Even if one measured diameter is within tolerance, a thread can cause difficulties during assembly due to deviations in the pitch or flank angle.
The so-called virtual pitch diameter plays an important role here.
The GO side checks the interaction of the thread geometry. In addition to the pitch diameter, it also takes account of the effects of pitch, flank-angle and form deviations. If these deviations are too large, the GO gauge may not screw in completely.
The NO-GO side checks the opposite limit of the simple pitch diameter. It therefore shows whether the internal thread has been made too wide or the external thread too small.
The inspection therefore provides a clear accept-or-reject decision, but no specific measured value for the magnitude of a deviation.
Not all thread dimensions are fully covered. If the inspection requirements call for it, the minor diameter of an internal thread must be checked separately, for example with a plain limit plug gauge. The same applies to the major diameter of external threads.
“A thread is not automatically within tolerance just because a screw can be inserted. GO and NO-GO gauges allow us to check directly on the finished component whether the relevant tolerance limits have been met. This is quick and provides a clear accept-or-reject decision. This is particularly important in series production so that faulty threads are not discovered only during assembly.”
What should you do if the thread inspection does not work as expected?
An unusual inspection result does not immediately mean that the manufactured thread is scrap. The key is to narrow down the cause correctly.
The GO side gets stuck
First check that the correct thread gauge has been used and that the thread is free from chips, burrs or damage. If the gauge still gets stuck after cleaning, dimensional or geometrical deviations may be present. The thread must then be inspected more closely.
The NO-GO side screws in too far
For metric ISO threads, the inspection has not been passed if the rejection gauge engages by more than two turns. First check the thread designation and tolerance class of the gauge. If these match and the inspection result is reproducible, the permissible pitch-diameter limit may have been exceeded.
Only the GO side is used for the inspection
A fully inserted GO gauge alone is not sufficient. The NO-GO inspection is also required in order to check the opposite tolerance limit.
A matching screw does not replace this inspection either. It only shows that this mating part can be assembled, not whether the specified tolerances have been met.
Why are cleanliness and temperature important during thread inspection?
Even small chips, dirt particles or burrs can affect the inspection result. Both the workpiece and the gauge should therefore be clean and free from damage before inspection.
The thread gauge itself is also subject to a certain amount of wear. In particular, the GO side is fully inserted during every inspection and is therefore exposed to mechanical stress. Regular checks and calibration based on use help to ensure the reliability of the inspection equipment.
For frequently used metric coarse threads in sizes M3 to M24, VÖLKEL therefore also offers limit plug gauges with a DLC-coated GO side. DLC stands for Diamond-like Carbon, a diamond-like carbon coating. Its high surface hardness and low sliding friction reduce wear and therefore ensure a considerably longer service life for the acceptance side.
Temperature is another factor. The reference temperature in dimensional metrology is 20 °C. If the workpiece and gauge are at different temperatures, their dimensions may differ due to thermal expansion. This can affect the inspection result, particularly where tolerances are tight.
The materials used and their coefficients of thermal expansion also play a role. A workpiece that is still warm from machining should therefore not simply be inspected with a significantly cooler thread gauge.
Our practical tip: If a thread gauge produces an unexpected result, first check the marking on the inspection device, clean the workpiece and gauge, and pay attention to the temperature conditions. If the result remains unusual, the cause must be clarified on the thread or in the manufacturing process. This helps you avoid unnecessary rework and prevents faulty components from reaching assembly.
