Metric M Screw Threads: Profile, Designation and Tolerances: Limiting Dimensions

Table 11: Pitch-Diameter Tolerances (Td₂)

All dimensions in millimeters.

Note: Intermediate grades (3, 4, 5, 7, 9) are computed from the Grade 6 values using the ratios given above.

Limiting Dimensions — Internal Metric Threads (Table 12)

This is the table you use to verify that your tapped holes meet specification. Tolerance class 6H.

All dimensions in millimeters. ANSI/ASME B1.13M-1983 (R1995)

Limiting Dimensions — External Metric Threads (Table 13)

This is the table that controls your bolt and screw dimensions. Both 6g and 4g6g classes shown.

All dimensions in millimeters. ANSI/ASME B1.13M-1983 (R1995)

Formulas for Internal Thread Limiting Dimensions

$$\text{Min major dia.} = \text{basic major dia.} + EI \text{ (Table 7)}$$

$$\text{Min pitch dia.} = \text{basic major dia.} - 0.649519P + EI_{D_2} \text{ (Table 7)}$$

$$\text{Max pitch dia.} = \text{min pitch dia.} + T_{D_2} \text{ (Table 8)}$$

$$\text{Max major dia.} = \text{max pitch dia.} + 0.793857P$$

$$\text{Min minor dia.} = \text{min major dia.} - 1.082532P$$

$$\text{Max minor dia.} = \text{min minor dia.} + T_{D_1} \text{ (Table 9)}$$

Formulas for External Thread Limiting Dimensions

$$\text{Max major dia.} = \text{basic major dia.} - es \text{ (Table 7, absolute value)}$$

$$\text{Min major dia.} = \text{max major dia.} - T_d \text{ (Table 10)}$$

$$\text{Max pitch dia.} = \text{basic major dia.} - 0.649519P - es_{d_2} \text{ (Table 7)}$$

$$\text{Min pitch dia.} = \text{max pitch dia.} - T_{d_2} \text{ (Table 11)}$$

$$\text{Max flat form minor dia.} = \text{max pitch dia.} - 0.433013P$$

$$\text{Min rounded root minor dia.} = \text{min pitch dia.} - 0.616025P$$

$$\text{Min root radius} = 0.125P$$

Worked Example: M10 × 1.5 − 6g External Thread

Given: Basic major diameter = 10.000 mm, P = 1.5 mm, es = 0.032 mm (from Table 7 for g position, P = 1.5)

Step 1 — Max major diameter: 10.000 − 0.032 = 9.968 mm

Step 2 — Min major diameter: 9.968 − 0.236 (Td from Table 10, Grade 6, P = 1.5) = 9.732 mm

Step 3 — Max pitch diameter: 10.000 − 0.649519(1.5) − 0.032 = 10.000 − 0.974279 − 0.032 = 8.994 mm

Step 4 — Min pitch diameter: 8.994 − 0.132 (Td₂ from Table 11, Grade 6... using interpolation from the data) = 8.862 mm

Step 5 — Max flat minor diameter: 8.994 − 0.433013(1.5) = 8.994 − 0.649520 = 8.344 mm

Step 6 — Min rounded root minor diameter: 8.862 − 0.616025(1.5) = 8.862 − 0.924038 = 7.938 mm

✅ Cross-check with Table 13: d Max = 9.968 ✓, d Min = 9.732 ✓, d₂ Max = 8.994 ✓, d₂ Min = 8.862 ✓, d₁ Max = 8.344 ✓, d₁ Min = 7.938 ✓

The formulas work. You are no longer dependent on finding your exact size in a table.

Dimensional Effect of Coating — The 4× Rule

This is the rule that catches experienced engineers off guard. the practitioner learned it just in time.

On a cylindrical surface, coating changes the diameter by twice the coating thickness.

On a 60-degree thread, because the coating thickness is measured perpendicular to the thread surface while the pitch diameter is measured perpendicular to the thread axis, a uniformly coated flank changes the pitch diameter by four times the coating thickness.

Rules for External Thread with No Allowance for Coating

To determine gaging limits before coating for a uniformly coated thread:

1. Decrease maximum pitch diameter by 4× maximum coating thickness
2. Decrease minimum pitch diameter by 4× minimum coating thickness
3. Decrease maximum major diameter by 2× maximum coating thickness
4. Decrease minimum major diameter by 2× minimum coating thickness

Standard Coating Accommodation

For standard external tolerance classes 6g and 4g6g, size limits apply prior to coating. The external thread allowance may be used to accommodate coating thickness, provided the maximum coating thickness is no more than one-quarter of the allowance.

After coating, the thread is subject to acceptance using a basic (tolerance position h) size GO thread gage and a tolerance position g thread gage for minimum material (LO or NOT-GO).

Coating Example: M6 × 1 − 4h6h with 0.010 mm Minimum Coating

After coating designation: M6 × 1 − 4h6h − AFTER COATING

Before coating limits:

• Major dia: 5.780 − 5.940
• Pitch dia: 5.239 − 5.290

A no-allowance thread after coating must not transgress the basic profile and is subject to acceptance using a basic (tolerance position H/h) size GO thread gage.

Thread Designation System — Complete Guide

Standard Designation Format

Metric screw threads are identified by:

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

Examples Decoded

Rounded Root Designation

The M profile with a minimum root radius of 0.125P on the external thread is desirable for all threads but is mandatory for threaded mechanical fasteners of ISO 898/I property class 8.8 (minimum tensile strength 800 MPa) and stronger. No special designation is required for these threads.

Modified Crest Designation

When limits of size of the major diameter (external) or minor diameter (internal) are modified:

M6 × 1 − 4h6h MOD followed by the modified diameter limits.

Length of Thread Engagement — Classification Table

The engagement length determines whether you use normal, short, or long tolerance adjustments.

Table 6: Length of Thread Engagement (ISO 965/1, ANSI/ASME B1.13M)

All dimensions in millimeters.

Key rule for short LE: Reduce the pitch diameter tolerance of the external thread by one tolerance grade number.

Key rule for long LE: Increase the allowance (fundamental deviation) at the pitch diameter of the external thread.

MJ Profile — The Aerospace Thread That Changes Everything

This is where the practitioner's story pivots. The MJ profile is not simply an M profile with a different letter. It's a fundamentally different design philosophy for thread roots.

What Makes MJ Different

The MJ screw thread (ANSI/ASME B1.21M-1978) is intended for:

• Aerospace metric threaded parts
• Highly stressed applications requiring high fatigue strength
• "No allowance" applications

The MJ profile is the hard metric version similar to the UNJ inch thread (MIL-S-8879). Its defining features:

MJ Diameter–Pitch Combinations for Aerospace

Standard Thread Series for Aerospace Screws, Bolts, and Nuts (ANSI/ASME B1.21M-1978):

All dimensions in millimeters.

MJ Tolerance Classes

External threads (after all processing including coating/plating):

• One tolerance class only: 4h6h
• Tolerance position h = no allowance
• Pitch diameter tolerance = grade 4
• Major diameter tolerance = grade 6

For coated/plated external threads (pitches ≤ 2 mm):

• Before processing: 4g6g
• Tolerance position g provides allowance for coating only
• For pitches > 2 mm: special allowances apply

Internal threads (after all processing):

For coated/plated internal threads (pitches ≤ 2 mm):

MJ Designation

Format: MJ[size] × [pitch] − [tolerance class]

Example: MJ6 × 1 − 4h6h

Unless otherwise specified, the thread helix is right hand.

Critical Interchangeability Warning

M profile and MJ profile threads 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 means you cannot freely substitute M profile internal threads for MJ profile internal threads in an aerospace assembly. The MJ internal thread has its minor diameter specifically truncated to clear the larger MJ root radius.

Trapezoidal Metric Thread — DIN 103 Power Transmission

While the practitioner's aerospace project uses the MJ profile, her colleague the practitioner Petrov is designing a lead screw assembly for a CNC positioning table. the practitioner needs a thread designed not for clamping force, but for smooth, efficient power transmission — and that means a completely different thread form.

The Trapezoidal Thread Form

The trapezoidal metric thread (DIN 103) has a 30-degree flank angle (compared to 60 degrees for M and MJ profiles). This reduced angle creates:

• Lower friction during axial movement
• Higher efficiency in converting rotational motion to linear motion
• Greater load-carrying capacity on the thread flanks

Trapezoidal Thread Formulas

Where a = clearance and b = backlash (see table below).

Root radii: Rounded to r = 0.25 mm for pitches 3–12 mm inclusive, and r = 0.5 mm for pitches 14–26 mm inclusive, for power transmission applications.

Complete Trapezoidal Thread Dimension Table (DIN 103)

All dimensions in millimeters.

The Master Decision Matrix: M vs. MJ vs. Trapezoidal

You're staring at a design. You need a metric thread. Which one?

Engineering takeaway

the practitioner didn't just learn a few new tables. She rebuilt her mental model of metric threads from the ground up. Here's what changed:

1. The tolerance system is not arbitrary — it's algebraic. Every dimension derives from formulas using basic size, fundamental deviation, and tolerance grade. Once you understand the formula structure, you can calculate dimensions for any size, even those not listed in standard tables.

2. The 4× coating rule is non-negotiable. On a 60-degree thread flank, coating changes the pitch diameter by four times the coating thickness. Forget this, and your coated threads won't gage. Remember it, and you can confidently specify coated threads for any environment.

3. MJ is not "M with extras" — it's a different safety philosophy. The controlled root radius of 0.15P to 0.18P in MJ threads is not a manufacturing convenience. It's a deliberate fatigue-life design feature that reduces stress concentration at the most failure-prone location in any threaded fastener.

4. Trapezoidal threads serve a fundamentally different purpose. Using an M profile thread for power transmission is like using a wrench as a hammer — it might work, but you're fighting the geometry. The 30-degree flank angle of trapezoidal threads is specifically optimized for axial force transfer with minimal friction.

5. Designation is a language, not a label. Every character in a metric thread designation carries specific engineering meaning. M6 × 1 − 4g6g tells you the profile, the diameter, the pitch, and both tolerance classes in nine characters. Learn to read it fluently, and you'll never misspecify a thread again.

Quick-Reference Formula Card

Cut this out. Tape it to your desk. Use it every day.

M Profile Geometry (ISO 68)

External Thread Limits

Internal Thread Limits

Coating Effect

Your Next Step

You now have every table, formula, tolerance value, and design principle needed to specify metric screw threads at a professional level — whether you're working with general-purpose M profile fasteners, aerospace-grade MJ threads, or trapezoidal power transmission screws.

Here's what to do next:

1. Print the Quick-Reference Formula Card and keep it where you do your design work.
2. Verify your current projects — are you using the right tolerance class for the application? Is 6H/6g actually appropriate, or should you be specifying 4g6g for a tighter fit?
3. Check your coated thread specifications — are you accounting for the 4× pitch diameter effect?
4. If you work with aerospace or high-fatigue applications, confirm that MJ profile threads are specified where the design requires controlled root radii.

The question that separates competent engineers from exceptional ones:

When was the last time you verified that your thread specification actually matches the stress, fatigue, and environmental requirements of your application — rather than defaulting to "what we've always used"?

This reference is based on ANSI/ASME B1.13M-1983 (R1995) for M Profile threads, ANSI/ASME B1.21M-1978 for MJ Profile threads, and DIN 103 for Trapezoidal Metric threads. All dimensional data preserved from original engineering handbook sources. Verify against current editions of these standards for production use.