Thread Gages, Acceptance and Inspection: The GO / NOT GO Decision Matrix

The GO / NOT GO Decision Matrix

This quick-reference matrix captures the accept/reject logic for every common gaging scenario:

Internal Thread Gaging (Using Plug Gages)

External Thread Gaging (Using Ring and Snap Gages)

Special Considerations: What the Standard Warns You About

Ductile and Thin Materials

The three-turn rule for NOT GO gages assumes standard material rigidity. Special requirements such as exceptionally thin or ductile material, or a small number of threads, may necessitate modification of the three-turn practice. If you are gaging thin-wall tubing, soft aluminum, or parts with very short thread lengths, work with your quality team to establish appropriate acceptance criteria.

The Wear Factor

Starting threads on both NOT GO (HI) plugs and NOT GO (LO) rings are subject to greater wear than the remaining threads. This wear, combined with incomplete product threads at the entry, permits further gage engagement than would occur with a new gage.

This means your NOT GO gages become more permissive as they wear. Parts that should be rejected begin passing the worn NOT GO gage. This is why regular verification against master setting gages is not optional—it is the only defense against gradual tolerance drift.

Multiple Gage Types and Borderline Parts

When a product thread is near the rejection limit, it is possible for one gage type to accept it while another rejects it. Similarly, two gages of the same type at opposite extremes of the gage tolerance band can produce different decisions on the same borderline part.

The standard resolves this ambiguity with a clear rule: A product screw thread is considered acceptable when it passes a test by any of the permissible gages for the gaging system that are within the tolerances.

The Complete Gage Ecosystem: How Everything Connects

Here is how the entire system fits together, from the governing standards down to the shop floor:

ANSI/ASME B1.2-1983 (R1991)
    │
    ├── Defines gage types (plug, ring, snap, indicating, setting)
    ├── Specifies thread forms of gages (Table 1)
    ├── Specifies W and X tolerances (Table 2)
    ├── Specifies plain gage tolerances (Table 3)
    ├── Provides computing constants (Table 4)
    └── Provides limit formulas (Table 5)
            │
            ├── EXTERNAL THREAD GAGES
            │     ├── GO Ring (Formula 1: PD = Max PD ext., tol. minus)
            │     ├── NOT GO (LO) Ring (Formula 3: PD = Min PD ext., tol. plus)
            │     ├── GO Plain (Formula 5: Major = Max major ext., tol. minus)
            │     └── NOT GO Plain (Formula 6: Major = Min major ext., tol. plus)
            │
            ├── INTERNAL THREAD GAGES
            │     ├── GO Plug (Formula 8: PD = Min PD int., tol. plus)
            │     ├── NOT GO (HI) Plug (Formula 10: PD = Max PD int., tol. minus)
            │     ├── GO Plain (Formula 11: Minor = Min minor int., tol. plus)
            │     └── NOT GO Plain (Formula 12: Minor = Max minor int., tol. minus)
            │
            ├── SETTING PLUGS (W Tolerance)
            │     ├── GO (Full & Truncated forms, Formulas 13-15)
            │     └── NOT GO (LO) (Full & Truncated forms, Formulas 16-18)
            │
            └── SETTING RINGS (W Tolerance)
                  ├── GO (Formulas 19-20)
                  └── NOT GO (HI) (Formulas 21-22)

Practical Application: Working Through a Real Example

Let's work through the gage dimensions for a common thread: ¼–20 UNC-2A.

Given data from Unified Screw Thread tables:

• Pitch: $p = 0.050000$ inches (20 TPI)
• $H = 0.866025 \times 0.050 = 0.043301$ inches
• $H/2 = 0.021651$ inches
• $H/4 = 0.010825$ inches

From the product thread tables (Class 2A external):

• Maximum pitch diameter (external): 0.2164 inches
• Minimum pitch diameter (external): 0.2127 inches
• Maximum major diameter (external): 0.2489 inches
• Minimum major diameter (external): 0.2408 inches

GO Thread Ring Gage (Formula 1):

$$\text{GO Pitch Diameter} = 0.2164 \text{ inches (max PD of external thread)}$$

Gage tolerance is minus (W or X, depending on gage class).

For X tolerance at 20 TPI, ≤½ in. diameter: PD tolerance = 0.0003 in.

$$\text{GO Ring PD range} = 0.2164 \text{ to } (0.2164 - 0.0003) = 0.2161 \text{ inches}$$

NOT GO (LO) Thread Ring Gage (Formula 3):

$$\text{NOT GO (LO) Pitch Diameter} = 0.2127 \text{ inches (min PD of external thread)}$$

Gage tolerance is plus.

$$\text{NOT GO Ring PD range} = 0.2127 \text{ to } (0.2127 + 0.0003) = 0.2130 \text{ inches}$$

GO Minor Diameter (Formula 2):

$$\text{GO Minor Dia.} = 0.2164 - 0.021651 = 0.19475 \text{ inches}$$

NOT GO (LO) Minor Diameter (Formula 4):

$$\text{NOT GO Minor Dia.} = 0.2127 - 0.010825 = 0.20188 \text{ inches}$$

This example demonstrates the key principle: gage tolerances always fall within the product tolerance zone. The GO gage can only be smaller than the maximum-material product dimension. The NOT GO gage can only be larger than the minimum-material product dimension. The gage never makes the acceptance envelope wider than the product specification allows.

The Universal Takeaway: Why This Matters to You

the practitioner's story is not unique. Variations of it play out every day in machine shops, inspection labs, and assembly facilities across every industry that touches threaded fasteners.

The thread gage system—with its W and X tolerances, its GO/NOT GO logic, its setting plugs and reference gages—is one of the most elegant quality assurance architectures ever devised. It transforms the impossibly complex geometry of a screw thread into a binary decision: pass or fail.

But that elegance depends entirely on the discipline of the people who use it.

If you are a machinist: Understand that the gage on your bench is only as reliable as its last calibration. Verify before you trust.

If you are a quality engineer: Build and enforce the three-tier calibration hierarchy. Reference verifies inspection. Inspection verifies product. Working gages bridge the gap between inspections.

If you are a design engineer: Specify thread classes and gaging systems deliberately. The tighter the class, the more expensive the gaging infrastructure required to verify it. Design to the loosest class that meets functional requirements.

If you are a procurement specialist: When you buy thread gages, you are not buying pieces of hardened steel. You are buying the precision that separates conforming product from scrap. Buy from calibrated sources, and demand certificates traceable to national standards.

Quick-Reference Card: Thread Gage Essentials

Your Next Step

Pull one thread gage off your shop floor right now. Check its calibration record. Verify it against its setting plug or setting ring. If you cannot trace it back to a reference standard—if there is even a single broken link in the chain—you do not know whether the threads you verified last week are actually in tolerance.

That is not a quality system. That is hope.

And as the practitioner learned at 2:14 AM on a Tuesday, hope is not a gaging strategy.

What is your current thread gage calibration interval? When was the last time you verified your NOT GO gages against their setting plugs? Share your approach in the comments—or better yet, go check your gages right now.