Metric M Screw Threads: Profile, Designation and Tolerances: the practitioner

The Definitive Engineering Reference You'll Use for the Rest of Your Career

One wrong pitch. One mismatched tolerance class. That's all it took to ground an entire fleet of aircraft for six weeks—and cost a manufacturer 2.3 million units of value in warranty claims, lost contracts, and emergency rework.

The engineer who made the call? She knew metric threads "well enough." She'd been specifying M profile fasteners for a decade. But when the project required MJ profile aerospace threads with controlled root radii, she defaulted to what she'd always done—and the fatigue cracks started appearing at 60% of the expected service life.

This is the guide that would have saved her.

What You'll Master in This Guide

• The complete M Profile system — ISO 68 basic profile, crest/root geometry, and every symbol you need to read a thread specification
• Coarse and fine pitch diameter–pitch combinations — the exact tables from ANSI/ASME B1.13M-1983 (R1995)
• The tolerance system decoded — grades, positions, classes, and how they create fits
• Limiting dimensions — complete internal and external thread tables for standard sizes
• Coating and plating rules — how a uniform flank coating changes pitch diameter by 4× the thickness
• Formulas for calculating every dimension — so you never depend on a table that doesn't have your size
• MJ Profile for aerospace — why a controlled root radius changes everything for fatigue life
• Trapezoidal metric threads (DIN 103) — the power transmission workhorse, with full dimensional data
• A master decision matrix — M vs. MJ vs. Trapezoidal, so you pick the right thread the first time

the practitioner: "Metric Threads Are Simple, Right?"

Meet the practitioner, a mechanical design engineer at a mid-size contract manufacturing firm. For ten years, she's been the go-to person for fastener specifications. M8 × 1.25? She can recite the pitch diameter in her sleep. M12 × 1.75? She's tapped thousands of them.

the practitioner's confidence isn't misplaced. She understands the 60-degree symmetric profile, the relationship between pitch and lead, and why you don't mix coarse and fine threads in the same assembly. She's good.

But "good" has a ceiling. And the practitioner is about to hit it.

Her company just landed a contract for structural brackets on a regional aircraft. The specification calls for MJ6 × 1 − 4h6h fasteners with a minimum root radius of 0.15P, and the tolerance system is unlike anything she's seen in general-purpose fastener work.

She stares at the callout. The "J" is unfamiliar. The tolerance class notation looks similar to what she knows—but she has a nagging feeling that "similar" and "identical" are dangerously different words in aerospace.

She's right to worry.

The Foundation: ISO 68 Basic Profile and the M Thread System

Before the practitioner can understand what makes MJ threads special—and what makes trapezoidal threads an entirely different animal—she needs to rebuild her understanding of metric threads from the ground up. Not the shortcut version. The real version.

What Is the M Profile?

The American National Standard ANSI/ASME B1.13M-1983 (R1995) describes the metric M profile thread system. It's a 60-degree symmetrical screw thread based on the ISO 68 basic profile, and it's the global standard for general fastening in mechanisms and structures.

The M profile is not just an American standard wearing a metric hat. It's in full agreement with ISO screw standards and resolutions, which means threads produced to this standard are fully interchangeable with threads conforming to any national standard based on ISO 68 basic profile and ISO 965/1 tolerance practices.

Critical interchangeability note: M profile and MJ profile threads (ANSI/ASME B1.21M) will assemble with each other. However, external MJ threads will encounter interference on the root radii with internal M thread crests when both threads are at maximum material condition. This is not a trivial issue—it's the kind of detail that separates a functional assembly from a seized joint.

How M Profile Compares to Inch Threads

If you're transitioning from the Unified inch system, here's the mapping that matters:

This comparison matters enormously. If you're replacing an inch-system joint with a metric one "of equivalent class," you need to know that 6H/6g gives you a looser fit than the 2A/2B you're replacing. Specifying 4g6g or 4H5H/4h6h may be the correct choice.

The Symbols That Govern Every Dimension

You cannot read a metric thread specification without understanding the symbol system. These are not arbitrary letters—they are a precise language defined by ANSI/ASME B1.13M.

Master Symbol Reference

The capitalization rule: Uppercase letters always refer to internal threads (nuts). Lowercase letters always refer to external threads (bolts). This convention is universal across the ISO metric system and violating it on a drawing is a guaranteed path to manufacturing errors.

Key Definitions You Must Know

These aren't textbook definitions you can skim. Each one represents a concept that, if misunderstood, leads directly to rejected parts or field failures.

• Allowance — The minimum nominal clearance between a prescribed dimension and its basic dimension. Numerically equal to the absolute value of the ISO term "fundamental deviation." This is the intentional gap built into the system.

• Basic Thread Profile — The cyclical outline in an axial plane of the permanently established boundary between external and internal threads. All deviations are measured with respect to this boundary.

• Fundamental Deviation — The deviation (upper or lower) closest to the basic size. For external threads, it's the upper deviation es. For internal threads, it's the lower deviation EI. This is the starting point from which tolerances are applied.

• Tolerance Class — The combination of a tolerance position with a tolerance grade. It specifies the allowance and tolerance for pitch and major diameters of external threads, and pitch and minor diameters of internal threads.

• Tolerance Grade — A numerical symbol designating the magnitude of tolerances applied to crest and pitch diameters.

• Tolerance Position — A letter symbol designating the position of the tolerance zone relative to the basic size. This position provides the allowance (fundamental deviation).

Failure trigger and engineering context

the practitioner receives the full specification package for the aircraft bracket project. The thread callouts are clear: MJ6 × 1 − 4h6h for the external threads, with a mandatory root radius requirement.

She opens her handbook to the M profile tables. The M6 × 1 data is there—she can see the pitch diameter, the minor diameter, the tolerance values. But the "MJ" designation sends her to a different section entirely, and what she finds there changes everything she thought she knew about thread geometry.

The MJ profile has a controlled root radius of 0.15P to 0.18P on the external thread. The standard M profile? Its minimum root radius is 0.125P, and it's not controlled in the same way. The internal MJ thread has its minor diameter truncated to accommodate the external thread's maximum root radius.

the practitioner realizes that if she had specified standard M profile threads for this application, the root geometry would have been wrong. The stress concentration at the root would have been higher. Under cyclic loading—exactly the kind of loading an aircraft bracket sees—fatigue cracks would initiate at the thread roots far earlier than the design life requires.

This is exactly the failure mode that grounded the fleet in the story at the top of this guide.

The M Profile Design Geometry — Complete Technical Breakdown

Basic ISO 68 Profile

The basic M thread profile (ISO 68) is the starting point for all metric screw thread design. It defines a 60-degree included angle between thread flanks, with specific proportions governing every dimension of the thread form.

Fundamental geometric relationships (all dimensions expressed as multiples of pitch P):

M Profile Data Table (ANSI/ASME B1.13M)

This table gives you the exact calculated values for every standard pitch. Keep this table permanently accessible in your shop or design office.

Crest and Root Form

External thread crest: Flat, permitting corner rounding. Truncated 0.125H (= 0.108253P) from a sharp crest.

Internal thread crest: Flat. Truncated 0.25H (= 0.216506P) from a sharp crest.

External thread root: The root profile must lie within the tolerance zone. For the rounded root thread, the profile must be a continuous, smoothly blended non-reversing curve, no part of which has a radius of less than 0.125P, and which is tangential to the thread flank. The profile may comprise tangent flank arcs joined by a tangential flat at the root.

Internal thread root: Must not be smaller than the basic profile. The maximum major diameter must not be sharp.

Why this matters to the practitioner: The 0.125P minimum root radius of the M profile is an uncontrolled minimum. The actual root form can vary widely. For the MJ profile her aerospace project requires, the root radius is controlled at 0.15P to 0.18P—a deliberately larger, more precisely defined radius that dramatically reduces stress concentration and improves fatigue life.

Coarse Pitch Thread Series — The First Choice for Design

The standard M profile screw thread series for general-purpose equipment and mechanical fasteners is the coarse thread series. These diameter/pitch combinations are the preferred sizes and should always be your first choice.

Table 4: Coarse Pitch Diameter–Pitch Combinations (ANSI/ASME B1.13M)

All dimensions in millimeters.

* For high strength structural steel fasteners only.

Table 5: Fine Pitch Diameter–Pitch Combinations (ANSI/ASME B1.13M)

Fine pitch threads are used when you need reduced vibration loosening, finer adjustment, or higher tensile strength at a given diameter. They are the second choice after coarse pitch.

All dimensions in millimeters.

† M12 × 1.5 is only for wheel studs and nuts.

How the System Works

The ISO metric tolerance system is built on limits and fits. The limits of the tolerances on mating parts, together with their allowances (fundamental deviations), determine the fit of the assembly. Internal threads are equivalent to holes; external threads are equivalent to shafts.

Key concepts in order:

1. Basic Size — The zero line. The interface where two mating parts have a common reference at the h/H tolerance position (zero fundamental deviation).

2. Upper Deviation (ES/es) — The algebraic difference between the maximum limit of size and the basic size. Designated "écart supérieur" (ES for internal, es for external).

3. Lower Deviation (EI/ei) — The algebraic difference between the minimum limit of size and the basic size. Designated "écart inférieur" (EI for internal, ei for external).

4. Fundamental Deviations (Allowances) — The deviations closest to the basic size. By convention, the external thread lies below the zero line and the internal thread lies above it (except for interference fits). This makes the fundamental deviation negative for external threads and positive for internal threads.

5. Fits — Determined by the fundamental deviations assigned to mating parts. Can be clearance, transition, or interference.

Tolerance Grades

Each grade provides numerical values for various nominal sizes. The system uses these grades for four screw thread parameters:

Grade relationships (all based on Grade 6 as the reference):

Major Diameter Tolerance of External Thread (Td):

• Grade 4 = 0.63 × Td(6)
• Grade 8 = 1.6 × Td(6)

Pitch Diameter Tolerance of External Thread (Td₂):

• Grade 3 = 0.5 × Td₂(6)
• Grade 4 = 0.63 × Td₂(6)
• Grade 5 = 0.8 × Td₂(6)
• Grade 7 = 1.25 × Td₂(6)
• Grade 8 = 1.6 × Td₂(6)
• Grade 9 = 2.0 × Td₂(6)

Tolerance Positions

Capital letters for internal threads, lowercase for external threads.

How to Read a Tolerance Class Designation

The tolerance grade is given first, followed by the tolerance position:

• 4g = Grade 4, position g
• 5H = Grade 5, position H

To designate the full tolerance class, the pitch diameter grade and position is shown first, followed by the crest diameter grade and position:

• 4g6g = Pitch diameter is Grade 4 position g; Major diameter is Grade 6 position g (external thread)
• 5H6H = Pitch diameter is Grade 5 position H; Minor diameter is Grade 6 position H (internal thread)

If both are identical, you don't repeat: 4g alone stands for 4g4g; 5H alone stands for 5H5H.

Designating a fit: Internal thread class / External thread class, separated by a slash:

• M6 × 1 − 6H/6g
• M6 × 1 − 6H/4g6g

Fundamental Deviation (Allowance) Table

This is the table that controls how much clearance exists between mating threads before tolerances are even applied.

Table 7: Allowance (Fundamental Deviation) — ANSI/ASME B1.13M

All dimensions in millimeters. Allowance is the absolute value of fundamental deviation.

Engagement length adjustments:

* Position h applies to maximum material functional size (GO thread gage) for plated 6g and 4g6g class threads.

Pitch Diameter Tolerances — Internal Threads (TD₂)

Table 8: Pitch-Diameter Tolerances of Internal Metric Threads

All dimensions in millimeters. (ISO 965/1, ANSI/ASME B1.13M)

Minor Diameter Tolerances — Internal Threads (TD₁)

Table 9: Minor Diameter Tolerances of Internal Metric Threads

All dimensions in millimeters.

Major and Pitch Diameter Tolerances — External Threads

Table 10: Major Diameter Tolerances (Td)

All dimensions in millimeters.