GO and NO-GO gauges check different things. The GO gauge is a full-form gauge based on Taylor’s Principle — it simulates the mating part and checks all thread elements at once. The NO-GO gauge is a single-element gauge — it checks only the pitch diameter at the maximum material condition.
The GO gauge action is different depending on whether you are inspecting an internal or external thread. For internal threads (tapped holes, nuts), you use a thread plug gauge — insert the GO end into the hole. For external threads (bolts, shafts), you use a thread ring gauge — place the GO ring over the thread. In both cases, the GO gauge must pass fully under hand pressure.
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How to Use a GO Thread Plug Gauge
A GO thread plug gauge answers one question: does the internal thread have enough material removed, over its full length of engagement, for a mating screw to assemble? The check takes seconds, but the result is only reliable if the part and the gauge are prepared correctly. Use this sequence on the shop floor or at incoming inspection.
- Confirm the gauge matches the drawing. Read the marking on the handle or gauge body: thread size, pitch, tolerance class (for example M10×1.5-6H or 3/8-16 UNC-2B) and the word GO. A GO plug from a different class or pitch will give a meaningless result.
- Clean the part and the gauge. Blow out chips and coolant from the hole, and wipe the gauge with a lint-free cloth. Dirt on the flanks can make a good hole reject or scratch the gauge.
- Apply a light film of oil. A thin film of light oil reduces friction and wear. Avoid heavy grease that hides the feel of the gauge.
- Start the gauge square to the hole. Align the gauge axis with the thread axis and engage it by hand. Never start at an angle and never use a wrench or pliers on the handle.
- Rotate with finger pressure only. The GO plug should turn in smoothly without force. Light, even resistance is normal; binding is not.
- Check the full length of engagement. The GO plug must pass through the full thread length (or to the specified depth in a blind hole). Stopping partway is a reject, even if the first few turns felt free.
- Follow with the NO-GO check. The GO result alone does not prove the hole is in tolerance. Use the NO-GO member to confirm the pitch diameter is not oversize.
- Remove, wipe and store. Unscrew the gauge by hand, clean it, and return it to its case to protect the thread form.
Accept the part only when the GO plug enters the full length freely and the NO-GO member stays within its permitted entry. Under ISO 1502 the NO-GO plug may enter no more than two turns; under ASME B1.2 the NOT GO member may enter no more than three complete turns.
Why a GO Plug Fails a Good-Looking Hole
Many GO plug rejects involve holes that look perfect to the eye. The GO plug checks the whole thread form at once, so any local obstruction or form error stops it. Before blaming the tap or the gauge, work through the common causes below.
| Cause | What you see | Fix |
|---|
| Burrs at the hole entry | Gauge will not start, or binds on the first turn | Deburr or countersink the entry; check chamfer on the drawing |
| Chips or coolant packed in the hole | Gauge stops at the same depth on every attempt | Blow out and flush; review chip evacuation in blind holes |
| Worn tap | Gradual increase in GO rejects across a batch | Replace or resharpen the tap; log tap life by hole count |
| Plating or coating build-up | Holes passed before finishing, fail after | Tap oversize for the coating allowance or gauge before plating as agreed |
| Minor diameter undersize | Gauge binds on the crests even though flanks are clean | Check tap drill size; the minor diameter must not interfere with the GO crest |
| Pitch or lead error | Gauge enters a few turns, then locks | Check tap holder float, feed synchronisation and thread milling program |
| Flank angle or form error | Tight along the full length, not at one spot | Inspect tap geometry; confirm the correct thread form was cut |
| Damaged or dirty gauge | Known-good masters also reject | Clean and inspect the gauge; send for calibration if in doubt |
A useful rule: if the GO plug stops at a consistent depth, suspect chips or a blind-hole depth issue; if it binds evenly along the length, suspect tap wear, form error or coating. Because the GO gauge applies Taylor’s principle and checks all elements together, it is sensitive to errors that a pitch diameter measurement alone would miss. See Taylor’s principle of gauging for the reasoning.
GO Plug Gauge Design and Markings
The GO member is deliberately different from the NO-GO member. Its design follows directly from its job of simulating the mating screw.
Full thread form
A GO thread plug gauge carries the complete thread profile: full flanks, crest and root clearance matched to the basic form. This lets it detect errors in pitch diameter, lead, flank angle and minor diameter in one check.
Length
The GO member is long enough to cover the normal length of engagement, so cumulative lead error across several pitches is caught. The NO-GO member, by contrast, is short with truncated flanks, so it checks pitch diameter only.
Handle colour and identification
On double-ended plug gauges the GO end is typically the longer member, and many manufacturers use a green band or marking for GO and red for NO-GO. Colour conventions vary, so always read the engraved text rather than relying on colour alone.
Wear allowance
Because the GO plug rubs against every part it checks, its tolerance zone includes a wear allowance. A new GO plug sits inside the product limits, and it can wear down to a defined wear limit before it must be retired. The NO-GO member sees little contact and has no comparable wear allowance.
| Feature | GO plug member | NO-GO plug member |
|---|
| Thread form | Full profile | Truncated flanks |
| Length | Covers length of engagement | Short, about two to three pitches |
| What it checks | All elements together (virtual size) | Pitch diameter only |
| Wear allowance | Yes | No comparable allowance |
| Typical marking | Size, pitch, class, GO | Size, pitch, class, NO-GO or NOT GO |
GO Plug for 4H, 5H, 6H and 7H Threads
Buyers are often surprised that one GO plug covers several metric internal tolerance classes. For H-position internal threads the fundamental deviation is zero, so the minimum pitch diameter of the nut equals the basic pitch diameter in every class from 4H to 7H. The GO plug checks the minimum material condition limit at that lower boundary, so it is identical for 4H, 5H, 6H and 7H of the same size and pitch.
What changes between classes is the tolerance width, and therefore the maximum pitch diameter. That is the limit checked by the NO-GO plug, so each class needs its own NO-GO member. In practice a shop inspecting M8×1.25 in both 6H and 7H can share one GO plug and keep two NO-GO plugs. This does not apply to G-position threads such as 6G, where the fundamental deviation shifts the lower limit and a separate GO plug is required.
GO Plug Wear and Replacement
The GO plug is the member that wears. Each insertion removes a small amount of material from the flanks, and the gauge gradually drifts toward the product limit. A worn GO plug accepts holes that are slightly too small, which can lead to assembly failures at your customer.
- Set a calibration interval based on usage, not calendar time alone; high-volume lines may need checks every few months.
- Record pitch diameter readings at each calibration so you can see the wear trend and plan replacement.
- Retire the GO plug when it reaches its wear limit, or earlier if the flanks show damage.
- Replace the GO member separately where the design allows, rather than scrapping the NO-GO end.
For limit values and interval planning, read our guide to thread plug gauge wear limits.
GO Gauge vs GO Setting Plug
The two names sound similar but serve different purposes. A GO thread plug gauge is a working gauge used to inspect internal threads on parts. A GO setting plug is a reference gauge used to set or check a GO thread ring gauge, which in turn inspects external threads. Setting plugs are not used on production parts. Adjustable rings under ASME B1.2 are set with setting plugs, while solid rings, common for ISO metric threads, are checked against them. See do thread ring gauges need setting plugs for details.
Ordering a GO Plug Gauge
A complete specification avoids delays and wrong gauges. Include the following in your request:
- Thread standard and designation, for example M12×1.75-6H, 1/4-20 UNC-2B or 1/2-14 NPT
- Right-hand or left-hand thread
- GO only, NO-GO only, or a double-ended GO/NO-GO set
- Quantity and whether spare GO members are needed
- Calibration certificate requirement; an ISO/IEC 17025 certificate is available on request
- Drawing for any special or non-standard thread
Standard sizes ship from stock within 72 hours, custom gauges take 10–15 working days, and the MOQ is one piece. Browse our thread plug gauges range or send your specification through the thread gauge quote request page for a reply within 24 hours.
Frequently Asked Questions
Can a GO plug gauge be used on its own without a NO-GO gauge?
No. The GO plug only confirms the thread is not too small and will assemble with a mating screw. It cannot detect an oversize pitch diameter, which leads to a loose fit and reduced strip strength. Always pair the GO check with the matching NO-GO member for the same size, pitch and tolerance class to confirm the hole is within limits.
How much force should I use to turn a GO thread plug gauge?
Use finger pressure only. The GO plug should turn in smoothly by hand along the full length of engagement. If you need a tool or noticeable force, treat the result as a reject and investigate chips, burrs or tap wear. Forcing the gauge damages its flanks and accelerates wear, which reduces its accuracy on later parts.
Does one GO plug gauge work for both 6H and 7H threads?
Yes, for the same metric size and pitch. H-position internal threads have zero fundamental deviation, so the minimum pitch diameter is the same from 4H to 7H and the GO plug is identical. Only the NO-GO plug changes, because the maximum pitch diameter depends on the tolerance class. G-position threads such as 6G need a separate GO plug.
How do I know when a GO plug gauge is worn out?
Calibration is the only reliable way. The laboratory measures the pitch diameter and compares it with the wear limit for that size and class. Warning signs between calibrations include a GO plug that feels loose in known-good parts or visible flank damage. Recording readings over time lets you predict replacement before the gauge passes the limit.
What is the difference between a GO plug gauge and a GO setting plug?
A GO plug gauge inspects internal threads on production parts. A GO setting plug is a reference gauge used to set or check a GO thread ring gauge, which inspects external threads. Setting plugs should never be used on parts. If you use ring gauges, ask whether matching setting plugs are needed for your standard and calibration routine.
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