1. The Three Controlling Diameters

Every screw thread is defined by three diameters, and each plays a different role in whether a thread assembles and functions correctly:

Diameter	External thread (bolt)	Internal thread (nut)
Major	Outside diameter of the crests	Diameter at the roots
Pitch	Diameter where thread width = groove width (on an imaginary cylinder)	Same definition
Minor	Diameter at the roots	Diameter at the crests

Note the external/internal roles invert: an external thread’s major diameter mates with the internal thread’s minor diameter clearance, and vice versa — but it’s the pitch diameter that governs the actual meshing of the flanks.

2. Why Pitch Diameter Dominates

Pitch diameter is the functional diameter — it controls where the flanks of the two mating threads make contact and therefore controls fit, backlash, and thread strength. This is the dimension the Taylor principle (behind go/no-go gauging) is fundamentally protecting. Major and minor diameters matter mainly as clearance dimensions — they need enough margin that the crest of one thread doesn’t bottom out in the root of the other before the flanks engage. For this reason, tolerance systems put the most engineering attention into pitch diameter tolerance, while major/minor diameters get simpler, often one-sided, limits.

3. The ISO Metric Tolerance Class System (ISO 965-1 / ASME B1.13M)

A metric tolerance class is written as a number + letter (or number + letter + number + letter for a full designation):

[Grade][Position], e.g. 6g or 6H

Tolerance grade (the number)

Controls the magnitude (width) of the tolerance zone — analogous to an IT grade. Common grades run roughly 3 (tightest) to 9 (loosest), with 6 being the standard/medium grade used for general-purpose commercial fasteners.

Tolerance position (the letter)

Controls where the tolerance zone sits relative to the theoretical “basic size” profile — i.e., how much allowance (intentional clearance) is built in.

• External threads → lowercase letters: e, f, g, h
• h = zero allowance (tolerance zone starts exactly at basic size)
• g = small negative allowance (slightly undersized, most common)
• f, e = progressively more allowance — used for loose fits, hot-dip galvanizing clearance, high-temperature or dirty service
• Internal threads → uppercase letters: G, H
• H = zero allowance (tolerance zone starts exactly at basic size, most common)
• G = small positive allowance

Because h/H carry no allowance, h is the “tightest-start” external position and H is the “tightest-start” internal position — the pair 6g/6H is not “zero allowance on both sides,” it’s a small, standard allowance on the external thread paired with a zero-allowance internal thread, which is the industry-default commercial fit.

4. Reading a Full Designation

A complete metric thread tolerance class can specify pitch diameter tolerance and crest-diameter tolerance (major for external, minor for internal) separately:

M10 x 1.5 - 6g6g     → PD tolerance 6g, major-diameter tolerance 6g (same, so often just written "6g")
M10 x 1.5 - 6H5H     → PD tolerance 6H, minor-diameter tolerance 5H (tighter root control)
M10 x 1.5 - 4h6h     → tight-fit PD (4h) paired with a looser major-diameter class (6h)

When both halves of the designation match, the standard allows the shorthand of writing it once (6g instead of 6g6g). This is why you’ll most often see simply 6g (external) and 6H (internal) on drawings — it’s shorthand for matched pitch- and crest-diameter tolerance grades.

5. Length of Engagement Groups

Pitch diameter tolerance values aren’t fixed numbers — they scale with the length of thread engagement, grouped into:

• S – Short
• N – Normal (the default assumed if not stated; covers most practical engagement lengths)
• L – Long

A longer engagement length tightens the required pitch-diameter tolerance (more flank contact amplifies the effect of lead/angle error), so the actual micrometer values behind “6g” differ depending on whether engagement is S, N, or L — even though the callout looks the same.

6. Major and Minor Diameter: Asymmetric Treatment

• External thread: Major diameter has a defined tolerance zone (limits crest truncation/flatness); minor diameter is not independently toleranced — it’s a natural consequence of the tool cutting the specified pitch diameter and thread form, with only a reference minimum implied.
• Internal thread: Minor diameter (root) has a defined, controlled tolerance zone since it’s produced by drilling/tapping and directly affects thread strength and tap clearance; major diameter only has a specified minimum, with no enforced maximum — it’s simply the drilled hole before tapping and isn’t a functional fit surface.

This asymmetry reflects where each manufacturing process (cutting vs. drilling/tapping) actually needs control.

7. Equivalence to the Unified (Inch) System — ASME B1.1

ISO Metric	Unified (UN)	Fit description
4h / 4H (tight)	3A / 3B	Precision, minimal play, no allowance
6g / 6H	2A / 2B	General purpose — the default commercial standard
8g (looser)	1A / 1B	Loose fit for quick assembly, dirty/coated conditions

6g/6H is the metric equivalent of the ubiquitous 2A/2B inch classes — both are the default “unless otherwise specified” fit for commercial and industrial fasteners.

8. Practical Selection Guidance

• 6g/6H: Default choice for general fastening, commercial hardware, most structural and machine applications. Specify this unless a specific reason exists to deviate.
• Tighter classes (4h/4H, 4h6h, etc.): Precision assemblies, minimal backlash requirements, repeated assembly/disassembly where slop is undesirable, aerospace/instrument work.
• Looser classes (8g, etc.): Parts that will be hot-dip galvanized or otherwise coated/plated after threading (the allowance makes room for coating thickness), threads used in dirty or high-temperature environments, or applications needing fast, forgiving assembly.
• Always confirm whether a drawing’s tolerance class applies to both pitch and crest diameter or specifies them separately (e.g., 6H5H) — this matters most on internal threads where root condition affects fatigue life.

Governing Standards

• ISO 965-1 — ISO general-purpose metric screw threads, tolerances
• ASME B1.13M — Metric screw threads (US adoption of ISO system)
• ASME B1.1 — Unified inch screw threads, classes 1A/1B, 2A/2B, 3A/3B