M18 × 2.5 M profile External Thread Specifications
M18x2.5-6g
ASMESTD: ASME B1.13MSOURCE: Calculated (para 9.2(b): Tables 7, 9-13 and para 9.3)TYPE: External (bolt/screw)CLASS: 6gSERIES: M ISO 261 other sizes (Table 6)2nd choice (Table 6, p. 9)calculated, not printed in the standard
Major diameter
17.958 – 17.623
max – min
Pitch diameter
16.334 – 16.164
max – min
Minor diameter
15.251 – 14.624
max – min (min is a reference value)
Fundamental deviation
es −0.042
pitch 2.5 mm · position g
Thread profile
M18x2.5-6g: solid line = maximum material profile, dashed = minimum material, hatched = tolerance zone (Figs. 8 and 9), grey dashed = basic profile (Fig. 1). Drawn to scale; values in mm, diameters shortened.
Limits of size, class 6g
calculated, not printed in the standard.
| Dimension | Basic | Max | Min | Tolerance |
|---|---|---|---|---|
| Major diameter dOutside diameter over the crests | 18.000 | 17.958 | 17.623 | 0.335 |
| Pitch diameter d2Where thread and groove widths are equal; sets the fit | 16.376 | 16.334 | 16.164 | 0.170 |
| Minor diameter, flat root d1Max d1 (flat form); min = d3 of the rounded form, for reference | 15.294 | 15.251 | 14.624ref. | 0.627ref. |
| Rounded-form minor diameter d3d3 max and d3 min; used for tools and gaging | 15.169ref. | 14.624ref. | ||
| Deviations, pitch diameter es / eiUpper and lower deviation from the basic pitch diameter (Fig. 4) | — | −0.042 | −0.212 | — |
| Deviations, major diameter es / eiCrest diameter | — | −0.042 | −0.377 | — |
Dimensions in millimetres. Values are those of the standard (ASME B1.13M); this row is calculated. Where the standard prints a value that differs from its formula, the printed value is shown with a note.
Tolerance zones relative to the basic size, mm (Fig. 4)
Lead, flank angle and length of engagement
- Allowable lead variation ±δP
- 0.049 mmOver the length of engagement
- Equivalent change in functional diameter δd2
- 0.085 mmHalf the pitch diameter tolerance
- Allowable half-angle variation ±δα
- 1° 20′ Each 30° flank angle
- Length of engagement, normal LE
- 10.0 – 30.0 mmShort up to / long over: see all values
Lead source: calculated, para 10.1.5; half-angle source: Table 18; length of engagement: Table 7
Notes
- Normal tolerance, small allowance (para 5.5.1(a)); the usual bolt class, ~ inch 2A.
Mates
M18x2.5-6g screws into a thread of the same standard at the same size and pitch. The standard mating internal thread is class 6H. Check the fit with the standard pair →
Working out the mating options…
Different pitch or a different thread family never mate; the fit check runs over both tolerance zones.
Related thread sizes
Same diameter other pitch, and neighbouring sizes
All values for M18x2.5-6gevery column of the workbook row
| Designation | M18x2.5-6g |
|---|---|
| Alternate designation (all capitals, para 7.2.3) | M18X2.5-6G EXT |
| Pitch in designation | 2.5 |
| Thread form | M |
| Series | Coarse |
| Series name | ISO 261 coarse pitch, not a B1.13M standard size (Table 6) |
| External / internal | External |
| Class | 6g |
| Nominal size | M18 |
| Pitch P (mm) | 2.50 |
| Basic major dia D, d (mm) | 18.000 |
| Size category | 2nd choice (Table 6, p. 9) |
| Source table | Calculated (para 9.2(b): Tables 7, 9-13 and para 9.3) |
| Thread type | M ISO 261 other sizes (Table 6) |
| Tolerance position, pitch dia | g |
| Tolerance grade, pitch dia | 6 |
| Tolerance position, crest dia | g |
| Tolerance grade, crest dia | 6 |
| Fundamental deviation es / EI, pitch dia (mm) | -0.042 |
| Fundamental deviation es / EI, crest dia (mm) | -0.042 |
| Allowance |es| / EI (mm) | 0.042 |
| Major dia max (mm) | 17.958 |
| Major dia min (mm) | 17.623 |
| Major dia tolerance Td (mm) | 0.335 |
| Major dia status | Toleranced (d max, d min) |
| Basic pitch dia D2, d2 (mm) | 16.376 |
| Pitch dia max (mm) | 16.334 |
| Pitch dia min (mm) | 16.164 |
| PD tolerance Td2 / TD2 (mm) | 0.170 |
| Basic minor dia D1, d1 (mm) | 15.294 |
| Minor dia max (mm) | 15.251 |
| Minor dia min (mm) | 14.624 |
| Minor dia tolerance TD1 (mm) | 0.627 |
| Minor dia status | d1 max toleranced (flat form, para 9.3.2(e)); min is d3 min of the rounded form, for reference (Table 14 note 4) |
| Max minor dia, flat form d1 (mm) | 15.251 |
| Min minor dia, rounded form d3 (mm, reference) | 14.624 |
| Max minor dia, rounded form d3, para 9.3.2(f) (mm) | 15.169 |
| Upper deviation, pitch dia es / ES (mm) | -0.042 |
| Lower deviation, pitch dia ei / EI (mm) | -0.212 |
| Upper deviation, crest dia (mm) | -0.042 |
| Lower deviation, crest dia (mm) | -0.377 |
| LE short, up to (mm) | 10.0 |
| LE normal, over (mm) | 10.0 |
| LE normal, up to and incl. (mm) | 30.0 |
| LE long, over (mm) | 30.0 |
| LE source | Table 7 |
| Lead variation ± (mm) | 0.049 |
| Equivalent change in functional dia (mm) | 0.085 |
| Lead values source | calculated, para 10.1.5 |
| Half-angle variation ± | 1° 20′ |
| Half-angle source | Table 18 |
| Lead angle at basic PD (deg) | 2.7820 |
| Lead angle (deg min) | 2° 47′ |
| Tensile stress area As (mm²) | 192.473 |
| Thread angle (deg) | 60 |
| Flank angle (deg) | 30 |
| Sharp V height H (mm) | 2.165064 |
| Thread height h = 5H/8 (mm) | 1.353165 |
| Crest flat (mm) | 0.312500 |
| Root flat (mm) | 0.625000 |
| Sweep cut: start diameter (mm) | 17.958000 |
| Sweep cut: tip diameter (mm) | 15.251000 |
| Sweep cut: depth (mm) | 1.353500 |
| Sweep cut: profile width at start diameter (mm) | 2.187500 |
| Crest flat as modelled (mm) | 0.312383 |
| Check against formulas | Calculated with the standard's formulas; no printed limits for this designation. |
| Notes | Normal tolerance, small allowance (para 5.5.1(a)); the usual bolt class, ~ inch 2A. |
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ASME B1.13M: Metric Screw Threads: M Profile. Source: workbook data (2026-10-10). Confirm against your licensed copy of the standard before use.