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Grade I / II / III for Threads: How to Specify and Inspect Clearly | UJEN

Grade I / II / III (or 1 / 2 / 3) is used in some supply chains as an accuracy/fit classification, and it can be verified using limit thread gauges (plug gauges and ring gauges). The key risk is not whether it can be inspected—it can—but whether the grade language is anchored to a defined standard and gaging system. ISO metric acceptance is typically communicated through tolerance classes such as 6H/6g (per ISO tolerance and gaging standards), while Unified inch acceptance uses classes such as 2A/2B (per ASME standards). To prevent mismatched inspection assumptions, use Grade I/II/III only when both parties share the same standard reference; otherwise, specify ISO/ASME classes directly. UJEN stock items are commonly supplied under a general-purpose practice often referenced as Grade II in some supply chains, and we support ISO/ASME class requirements when stated on the RFQ/drawing, including pre- or post-plating gaging basis if plating is involved.

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Grade I / II / III for Threads: How to Specify and Inspect Clearly

Grade I / II / III for Threads: How to Specify and Inspect Clearly

grade-i-ii-iii-usage-guide

Grade I / II / III (or 1 / 2 / 3) is used in some supply chains as an accuracy/fit classification, and it can be verified using limit thread gauges (plug gauges and ring gauges). The key risk is not whether it can be inspected—it can—but whether the grade language is anchored to a defined standard and gaging system. ISO metric acceptance is typically communicated through tolerance classes such as 6H/6g (per ISO tolerance and gaging standards), while Unified inch acceptance uses classes such as 2A/2B (per ASME standards). To prevent mismatched inspection assumptions, use Grade I/II/III only when both parties share the same standard reference; otherwise, specify ISO/ASME classes directly. UJEN stock items are commonly supplied under a general-purpose practice often referenced as Grade II in some supply chains, and we support ISO/ASME class requirements when stated on the RFQ/drawing, including pre- or post-plating gaging basis if plating is involved.

Grade I / II / III Can Be an Inspection Language

Grade I / II / III (or 1 / 2 / 3) is used in some supply chains as an accuracy/fit classification for threads. It can be verified using limit thread gauges (plug gauges and ring gauges), just like ISO or Unified systems. The key is to anchor the grade language to a defined standard and gaging system so both parties inspect the same way.

Why a Standard Reference Matters

The risk with grade-only callouts is not “it cannot be inspected” — it can. The risk is that different parties may assume different gaging systems, different acceptance limits, or a different measurement basis (especially when plating is involved). This is why grade language should be linked to a standard reference whenever it is used for acceptance.

ISO 1 / 2 / 3 Language (Fit/Accuracy Context)

In some engineering references, ISO 1 / 2 / 3 is used to describe fit intent. A common example in practice is that ISO 2 is treated as the “normal” tap tolerance for internal threads (often referenced as 6H), while ISO 1 is tighter and ISO 3 is looser. If your project is specified to ISO acceptance, use ISO tolerance classes (such as 6H/6g) as the primary drawing language.

When Grade I / II / III Works Well

  • Both buyer and supplier share the same referenced standard and gaging system
  • The receiving inspection is explicitly aligned to the same grade-based gage set
  • The measurement basis is clear (pre-plating vs post-plating) if parts are plated

When to Use ISO / ASME Classes Instead

If your project specifies ISO or ASME acceptance for gauging and inspection, state the standard-system language directly: ISO tolerance classes (metric) or Unified classes (inch). This prevents translation losses.

  • ISO metric acceptance: M8 × 1.25 - 6H (internal) / 6g (external)
  • Unified acceptance: 1/4-20 UNC-2A / 1/4-20 UNC-2B

Recommended Writing Pattern (Grade as Optional Companion)

If your supply chain commonly uses Grade language, you may include it as an optional companion label, while still stating ISO/ASME classes when the project is specified to those standards. This keeps communication friendly without forcing every buyer to adopt one notation style.

  • Example (metric, ISO specified): M8 × 1.25 - II / 6H (internal) / 6g (external)
  • Example (inch, ASME specified): 1/4-20 UNC-2A / 2B (Grade language optional only if agreed)

UJEN Practice (Default and Custom)

UJEN stock items are commonly supplied under a general-purpose practice (often referenced as Grade II in some supply chains). If your project requires ISO tolerance classes or Unified classes for defined acceptance, please specify them on the RFQ/drawing. If parts are plated, state whether acceptance is based on pre-plating gauging or post-plating gauging; UJEN will align the process and inspection route to meet the stated requirement.

RFQ Checklist (Copy/Paste Ready)

  • Thread family: ISO Metric / Unified / Whitworth
  • Geometry: diameter × pitch (metric) or size – TPI – series (inch)
  • Acceptance language:
    • ISO specified: state ISO tolerance class (e.g., 6H/6g)
    • ASME specified: state Unified class (e.g., 2A/2B)
    • Grade I/II/III: allowed when anchored to an agreed standard and gaging system
  • If plated: define measurement basis (pre- vs post-plating) and any post-plating gage-pass requirement

Grade I / II / III for Threads: How to Specify and Inspect Clearly | UJEN

UJEN's adjustable handle screws and spring pull pins are engineered for repeatable positioning and secure fastening in space-limited assemblies. With 45 years of mold engineering expertise and ISO 9001 certification, UJEN delivers custom fastening solutions that streamline manufacturing processes, enhance product functionality, and ensure consistent quality across high-volume production. Contact UJEN today to explore how application-driven fastening design can optimize your machinery performance.

Beyond supplying standard parts, UJEN provides technical consultation and OEM/ODM co-development services that translate functional requirements into manufacturable fastening solutions. The company's development philosophy—guided by four core principles of Unconventional Innovation, Just Right Design, Experimental Engineering, and New Sustainable Future—positions UJEN at the intersection of fastening components, mold engineering, and application-driven design. Whether you need eco-friendly fasteners using recycled materials, titanium alloy screws for lightweight applications, or distinctive visual finishes through hydrographic printing, UJEN supports manufacturers in building products that assemble faster, operate more intuitively, and scale reliably in production while enhancing competitive advantage in their respective markets.