Metric M Screw Threads: Profile, Designation and Tolerances: External Metric Thread

External Metric Thread — M Profile Limiting Dimensions (Table 13)

Tolerance Classes 6g and 4g6g — ANSI/ASME B1.13M-1983 (R1995)

All dimensions in millimeters. es is an absolute value.

Continued — Large Sizes (6g and 4g6g):

Notes:

• d1 Max = Flat form minor diameter
• d3 Min = Rounded root minor diameter (reference dimension for tool design; not normally specified on drawings)
• For tolerance position h threads, add the absolute value of es to the maximum diameters listed

Metric Screw Thread Designations — The Complete System

the practitioner learned the hard way that a thread callout contains critical information compressed into a short string. Here is how to read — and write — every designation correctly.

Basic Format

M [nominal diameter] × [pitch] − [tolerance class]

Example: M6 × 1 − 4g6g

• M = Metric thread, M profile
• 6 = 6 mm nominal (major) diameter
• × 1 = 1 mm pitch
• 4g6g = Tolerance class (Grade 4 pitch dia / Grade 6 major dia, position g)

Designation Rules

Right hand thread (default): No suffix required.

M6 × 1 − 4g6g (22)

Left hand thread: Suffix with − LH

M6 × 1 − 5H6H − LH (23)

Identical tolerance classes: Do not repeat.

M6 × 1 − 6H    (means 6H6H)

All-capitals (computer/teletype): Add EXT or INT.

M6 × 1 − 4G6G EXT
M6 × 1 − 6H INT

Thread fit (mating pair): Internal class / external class.

M6 × 1 − 6H/6g
M6 × 1 − 6H/4g6g

Rounded root (special): Suffix with root radius value + R.

M42 × 4.5 − 6g − 0.63R

Modified crests: Suffix with MOD + modified diameter limits.

M6 × 1 − 4h6h MOD
Major dia = 5.745 − 5.925 MOD

Special threads: Suffix with SPL + full dimension limits.

M6.5 × 1 − 4h6h − SPL (22)
Major dia = 6.320 − 6.500
Pitch dia = 5.779 − 5.850
Minor dia = 5.163 − 5.386

Multiple-start threads:

M16 × L4 − P2 − 4h6h (TWO STARTS)
M14 × L6 − P2 − 6H (THREE STARTS)

Coated/plated threads:

M6 × 1 − 6h AFTER COATING
M6 × 1 − 6g AFTER PLATING

If no "AFTER COATING" or "AFTER PLATING" is specified, the tolerance class applies before coating per ISO practice.

Coarse Pitch Shorthand

International practice allows omitting the pitch for coarse-pitch threads (e.g., M14 instead of M14 × 2). However, ANSI/ASME B1.13M makes it mandatory to include the pitch in all designations to prevent misunderstanding.

Comparison of Maximum Metal Dimensions — Metric vs. Inch

This is the bridge table for shops running both systems. The metric M profile threads of tolerance class 6H/6g are intended for applications where the inch class 2A/2B has been used.

Key comparisons:

Interchangeability

• Threads produced to ANSI/ASME B1.13M are fully interchangeable with threads conforming to other national standards based on ISO 68 basic profile and ISO 965/1 tolerance practices.
• Threads produced to B1.13M should be mechanically interchangeable with those produced to ANSI B1.18M-1982 (commercial mechanical fasteners) of the same size and tolerance class. However, some parts accepted by conventional gages may be rejected by Double-NOT-GO gages required for B1.18M.
• M profile and MJ profile threads will assemble with each other. However, external MJ threads may encounter interference on root radii with internal M thread crests when both are at maximum material condition.

The MJ Profile — Aerospace-Grade Threads

When the practitioner's bridge situation demanded the highest fatigue strength, his stress engineer pointed him toward the MJ profile — the hard metric version of the UNJ inch thread (MIL-S-8879).

What Makes MJ Different

The MJ thread has a controlled root radius of 0.15P to 0.18P on the external thread (compared to 0.125P minimum for standard M profile). The internal thread minor diameter is truncated to accommodate this larger root radius.

Result: Significantly higher fatigue strength due to reduced stress concentration at the thread root.

Standard

ANSI/ASME B1.21M-1978 establishes the basic triangular profile for the MJ form, covering diameters from 1.6 to 200 mm.

Tolerance Classes

External threads (after all processing): 4h6h

• Tolerance position h = no allowance
• Pitch diameter tolerance = Grade 4
• Major diameter tolerance = Grade 6
• For coated threads with P ≤ 2 mm: tolerance class 4g6g before processing (position g provides allowance for coating)

Internal threads (after all processing):

• M1 through M5: 4H6H
• M6 and larger: 4H5H
• Tolerance position H = no allowance
• Pitch diameter tolerance = Grade 4 for all sizes
• For coated threads with P ≤ 2 mm: 4G6G (sizes ≤ 5 mm) or 4G5G (sizes ≥ 6 mm)

Aerospace Thread Series (ANSI/ASME B1.21M)

Designation: MJ6 × 1 − 4h6h

the practitioner's Transformation — The Lesson Learned

Six months after the bridge scare, the practitioner's shop had become unrecognizable.

Every CNC programmer had the M Profile data table (Table 3) taped to their workstation. The quality team ran pitch diameter checks against Table 12 and 13 limits on every batch. Procurement now required tolerance class certification from every fastener supplier, verified against ANSI/ASME B1.13M.

The coating calculation became standard operating procedure. No bolt left the shop without someone verifying that the allowance could accommodate the specified coating thickness per the quarter-allowance rule.

And those bridge bolts? Every last one was replaced with properly manufactured, properly gaged, properly coated M24 × 3 − 6g fasteners with verified rounded roots. The structure passed its fatigue inspection with flying colors.

The real transformation wasn't technical. It was cultural.

the practitioner stopped thinking of thread tolerances as paperwork and started thinking of them as engineering insurance. Every tenth of a millimeter on a pitch diameter represents a decision — a decision about safety, reliability, and professional integrity.

Your Quick-Reference Formula Card

Cut this section out and keep it where you work.

The Core Relationships

The Decision Matrix

What's Your Next Thread Decision?

Every fastener you specify, machine, or inspect carries a hidden story — a story written in tolerance grades and fundamental deviations. The question is whether you're reading that story, or ignoring it.

Here's your challenge:

Pull the last metric bolt drawing you worked with. Look at the tolerance class. Now check it against Tables 12 and 13 in this guide. Does the actual bolt fall within limits? Was the coating thickness accounted for? Is the engagement length classified as short, normal, or long — and was the tolerance grade adjusted accordingly?

If you can answer all three questions with confidence, you're operating at the level this industry demands.

If you can't — you now have every tool, table, and formula you need to get there.

The threads hold the world together. Make sure yours are right.

Reference Standard: ANSI/ASME B1.13M-1983 (R1995), ISO 965/1, ISO 68. All dimensional data in millimeters unless otherwise specified.