Thread Fit Language Alignment
Unify “what it means” before debating “what it passes”: ISO tolerance classes, Unified classes, and legacy grade terms
Most thread disputes are not caused by the thread itself, but by mismatched acceptance language. A drawing might show only “M8 × 1.25” or “1/4-20”, while the buyer’s inspection expects a specific gaging system, tolerance class, and plating basis. This page aligns the three major languages used in global sourcing: ISO metric tolerance classes (e.g., 6H/6g), Unified inch classes (e.g., 2A/2B), and legacy grade terms such as I/II/III used in some supply chains. The goal is not to claim perfect 1:1 equivalence across systems, but to prevent avoidable misunderstandings: define the standard family, declare the acceptance system, and state the measurement basis when plating is involved.
Why Thread “Fits” Become Disputes
Thread callouts can describe geometry (size and pitch) or they can describe acceptance (gaging system and tolerance). Problems happen when the drawing states only geometry while the receiving side assumes a specific acceptance language. Aligning terminology up front reduces rework, delays, and quality disputes.
Three Common Thread “Languages”
- ISO Metric: tolerance classes such as 6H (internal) / 6g (external) defined under ISO 965
- Unified Inch: classes such as 2A (external) / 2B (internal) defined under ASME B1.1
- Legacy Grade Terms: I / II / III used in some supply chains as general-purpose intent language
Important: No Perfect 1:1 Conversion
“Grade II” and “6H/6g” are not the same thing, and “2A/2B” is not a direct translation of ISO metric classes. They come from different standards and different gaging systems. Some combinations may assemble smoothly in practice, but acceptance criteria and measurement methods can differ.
ISO Metric: What 6H / 6g Actually Means
ISO tolerance classes combine a grade number and a deviation letter. For example, 6H is commonly used for internal threads and 6g for external threads in general-purpose design intent. ISO acceptance typically references ISO 965 (tolerance system) and ISO 1502 (gaging). If your project is specified to ISO standards, state the ISO tolerance class on the drawing/RFQ.
Unified Inch: What 2A / 2B Actually Means
Unified threads specify the series (UNC/UNF), then the class of fit. “A” indicates external threads; “B” indicates internal threads. Class 2 (2A/2B) is the most common general-purpose fit in industrial applications. Unified acceptance typically references ASME B1.1 (thread system) and ASME B1.2 (gaging).
Legacy Grade Terms (I / II / III): How to Treat Them
In some supply chains, I / II / III are used as shorthand for general-purpose intent. Because this language is not a single global standard, it should be treated as “procurement shorthand” rather than a complete acceptance definition. When a project is specified to ISO or ASME acceptance, the drawing should state the proper tolerance class or class of fit.
Practical Alignment Rules (Recommended)
- Always state the standard family: ISO Metric (M), Unified (UNC/UNF), or Whitworth (BSW/BSF)
- When ISO acceptance is required: state ISO tolerance class (e.g., 6H internal / 6g external, or customer-defined)
- When Unified acceptance is required: state Unified series and class (e.g., 1/4-20 UNC-2A/2B)
- If plated: define measurement basis (pre-plating gauging or post-plating gauging)
- If legacy grade language appears: treat it as intent; confirm acceptance system in RFQ
UJEN Practice (How We Avoid Misalignment)
UJEN supports ISO Metric, Unified (UNC/UNF), and Whitworth (BSW/BSF). For requests without a declared acceptance system, we align the callout language during RFQ to match your inspection and receiving requirements. If plated parts require post-plating gage-pass acceptance, we will quote based on the defined process route and inspection method.