ISO Metric Thread Tolerance Classes Explained (6H / 6g and Beyond)
How grade numbers and deviation letters work—so you can specify ISO intent without over-specifying
ISO metric thread tolerance classes (such as 6H for internal and 6g for external) are defined by a grade number plus a deviation letter. The grade number relates to the tolerance width, while the deviation letter sets where that tolerance zone sits relative to the basic profile. This matters because two callouts can share the same nominal size and pitch but behave differently in gauging feel, interchangeability, and plating risk. This page explains how ISO tolerance classes are structured, how to interpret common classes beyond 6H/6g, and how to write requirements clearly when a project is specified to ISO acceptance. UJEN can support ISO tolerance-class requirements when stated on drawings/RFQs, and will align measurement basis (pre- or post-plating) where plating is involved.
What an ISO Tolerance Class Represents
ISO metric thread tolerance classes are written as a number + a letter (for example: 6H or 6g). The number indicates the tolerance grade (tolerance width), and the letter indicates the fundamental deviation (the position of that tolerance zone relative to the basic profile).
A Simple Analogy (Parking Space)
Think of a tolerance class like a parking space system. The grade number is the parking space width: tighter grade means a narrower space and less room for variation. The deviation letter is where the space is painted relative to the centerline: it can be shifted slightly to one side. Two parking spaces can have the same width, but if one is shifted, your car ends up sitting in a different position. That is why the same “size” thread can feel different in assembly and gaging when the tolerance zone position changes.
Grade Number: Tolerance Width
The grade number (such as 4, 5, 6, 7, 8) corresponds to how wide the tolerance zone is. Lower numbers generally mean a tighter tolerance zone; higher numbers generally mean a wider tolerance zone. This affects both functional feel and how consistently parts pass gages across large production volumes.
Deviation Letter: Tolerance Zone Position
The deviation letter (such as H for internal threads, g for external threads) places the tolerance zone above, below, or around the basic profile. This is why two threads can have the same nominal size and pitch but different acceptance outcomes.
Common ISO Classes You May See
- 6H (internal): commonly used general-purpose internal thread class
- 6g (external): commonly used general-purpose external thread class
- 5H / 4H (internal): tighter intent (project-specific, not universal default)
- 7H / 8H (internal): looser intent (used where assembly margin is prioritized)
- 5g / 4g (external): tighter external thread intent
- 7g / 8g (external): looser external thread intent
Different classes are used depending on design intent, functional feel, and the acceptance system required by the buyer. Avoid assuming one class is “always correct” across all assemblies.
How to Specify ISO Acceptance Clearly
If a project is specified to ISO acceptance, state the ISO tolerance class on the drawing/RFQ and define the measurement basis when plating is involved. This aligns gaging, receiving inspection, and functional acceptance.
- ISO metric (example): M8 × 1.25 - 6H (internal) / 6g (external)
- If plated: define basis as pre-plating gauging or post-plating gauging
- If a specific gage system is referenced: align to ISO 1502
Avoid Over-Specifying When It Is Not Required
Some projects do not require a fully declared ISO gaging system on every drawing, especially when assembly performance is the primary goal and the buyer does not enforce a specific incoming gage system. In those cases, focus on clarity of size, pitch, and functional engagement, and align acceptance language during RFQ when needed.
UJEN Practice
If your project is specified to ISO acceptance, UJEN can supply according to the stated ISO tolerance class. If plating is involved, we will align the measurement basis (pre- or post-plating) and inspection routing to match your requirement. Buyers do not need to prescribe methods—state the acceptance requirement, and UJEN will align the production and inspection route.