Scope
ASME B1.13M covers metric screw threads of the M profile — the standard 60° thread used across the modern metric fastener system. It defines thread form, basic dimensions, standard pitches, tolerance classes, and designation conventions. The spec aligns with ISO 68-1 (thread profile) and ISO 965 (tolerances), making B1.13M threads interchangeable with threads made to the corresponding ISO standards.
Thread designation — how to read it
A standard metric callout:
M10 × 1.5 - 6g
- M — metric thread profile
- 10 — nominal diameter in mm (10.00 mm)
- × 1.5 — pitch in mm (1.5 mm between adjacent thread peaks)
- 6g — tolerance class (6 = grade, g = position, external thread)
For internal threads (tapped holes, nuts):
M10 × 1.5 - 6H
Same thread, same size, mating internal. Lowercase letter = external, uppercase = internal.
If no pitch is shown ("M10"), coarse pitch is assumed. Coarse pitch is the default on engineering drawings.
Coarse vs fine pitches
Unlike inch threads where UNC and UNF are distinct series with memorable TPI values, metric threads use explicit pitch numbers:
| Diameter | Coarse pitch | Common fine pitches |
|---|---|---|
| M3 | 0.5 | 0.35 |
| M4 | 0.7 | 0.5 |
| M5 | 0.8 | — |
| M6 | 1.0 | 0.75 |
| M8 | 1.25 | 1.0 |
| M10 | 1.5 | 1.25, 1.0 |
| M12 | 1.75 | 1.5, 1.25 |
| M14 | 2.0 | 1.5 |
| M16 | 2.0 | 1.5 |
| M20 | 2.5 | 1.5 |
| M24 | 3.0 | 2.0 |
| M30 | 3.5 | 2.0 |
| M36 | 4.0 | 3.0 |
Always confirm pitch. "M10" alone implies coarse pitch 1.5, but drawings for automotive and aerospace applications frequently call out M10×1.25 fine pitch — and these are not interchangeable. A coarse-pitch bolt will not thread into a fine-pitch nut.
Thread form geometry
B1.13M thread profile is identical to ISO 68-1:
- Thread angle (included): 60°
- Fundamental triangle height (H): 0.866025 × pitch
- Thread working height: 5/8 × H = 0.541266 × pitch
- Crest and root truncation: rounded or flat per the specific form
MJ is the metric counterpart to UNJ — a controlled-radius root profile for enhanced fatigue life in aerospace.
Tolerance classes — the metric fit system
Metric thread tolerance is described by two characters: a tolerance grade (number, 3 through 9) and a tolerance position (letter, e/f/g/h for external or E/F/G/H for internal).
External thread positions (lowercase)
- e — large fundamental allowance (looser fit)
- f — medium allowance
- g — small allowance (the default for most commercial production)
- h — zero allowance (tightest, for precision applications)
Internal thread positions (uppercase)
- G — small allowance
- H — zero allowance (standard commercial)
Tolerance grades (tighter as number decreases)
Grade 3 is the tightest (precision); grade 9 is the loosest. Grade 6 is the standard commercial production grade for both external and internal threads.
Common tolerance class combinations
| External × Internal | Character | Typical use |
|---|---|---|
| 6g / 6H | Standard commercial | Default; most bolts and nuts |
| 6g / 7H | Moderate clearance | Assembly under tight conditions |
| 4g6g / 5H6H | Precision | Aerospace, tight fit |
| 8g / 7H | Loose | Field assembly, tolerance to damage |
The important pair to remember: 6g / 6H is the default. Unless a drawing specifies otherwise, metric bolts and nuts are made to this tolerance class.
B1.13M vs B1.1 fit class conceptual comparison
While the systems differ in notation, they correspond:
| Inch (B1.1) | Metric (B1.13M) | Concept |
|---|---|---|
| 1A / 1B | 8g / 7H or 8g / 8H | Loose fit |
| 2A / 2B | 6g / 6H | Standard commercial |
| 3A / 3B | 4g6g / 5H6H or 4h / 5H | Precision |
Not identical, but operationally equivalent in most engineering contexts.
Coatings and thread fit
Same principles as inch threads: coatings add material to thread flanks and require tolerance consideration.
- Plain (uncoated): standard 6g/6H fits without modification
- Zinc electroplating (light): usually fits 6g/6H acceptably
- Hot-dip galvanizing: typically requires 6az (external) or 7H tap (internal) to accommodate coating thickness
- Zinc-flake coatings: usually fit 6g/6H acceptably
Pitch diameter gauging
Like inch threads, metric thread fit is governed by pitch diameter. ISO 1502 and ANSI/ASME B1.16M cover gauging of metric threads:
- Go / no-go plug gauges for internal threads
- Go / no-go ring gauges for external threads
- Three-wire measurement for precision applications
Applications
- All metric threaded fasteners (automotive, European and Asian OEM, global industrial)
- Metric tapped holes in machined components
- European-spec construction and industrial equipment
- Global aerospace using metric standards
- Any fastener whose designation begins with "M"
Related specifications
- ISO 68-1 — Metric thread profile (basis for B1.13M)
- ISO 965-1 / 965-2 — Metric thread tolerances (basis for B1.13M tolerance tables)
- B1.1 — Inch-series thread counterpart
- B1.20.1 — NPT pipe threads (not metric)
- B1.16M — Metric thread gauging
- ISO 898-1 / F568M — Metric property classes for bolts
- ISO 898-2 / F836M — Metric property classes for nuts
Documentation
California Fastener stocks fasteners manufactured to B1.13M / ISO 965 thread requirements in all standard diameters, coarse and fine pitches, and tolerance classes. Thread gauging documentation available on request for precision applications.