M18 × 2.5 M profile Internal Thread Specifications
M18x2.5-6H
ASMESTD: ASME B1.13MSOURCE: Calculated (para 9.2(b): Tables 7, 9-13 and para 9.3)TYPE: Internal (nut/tapped hole)CLASS: 6HSERIES: M ISO 261 other sizes (Table 6)2nd choice (Table 6, p. 9)calculated, not printed in the standard
Major diameter
18.000 min
max 18.585 (reference)
Pitch diameter
16.376 – 16.600
min – max
Minor diameter
15.294 – 15.744
min – max
Fundamental deviation
EI 0.000
pitch 2.5 mm · position H
Thread profile
M18x2.5-6H: 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 6H
calculated, not printed in the standard.
| Dimension | Basic | Max | Min | Tolerance |
|---|---|---|---|---|
| Major diameter DRoot of the internal thread; max is for tool design only | 18.000 | 18.585ref. | 18.000 | 0.585ref. |
| Pitch diameter D2Sets the fit | 16.376 | 16.600 | 16.376 | 0.224 |
| Minor diameter D1Crest of the internal thread (the bore) | 15.294 | 15.744 | 15.294 | 0.450 |
| Deviations, pitch diameter ES / EIUpper and lower deviation from the basic pitch diameter (Fig. 4) | — | +0.224 | 0.000 | — |
| Deviations, minor diameter ES / EICrest diameter | — | +0.450 | 0.000 | — |
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
- 0.065 mmOver the length of engagement
- Equivalent change in functional diameter δd2
- 0.112 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
- Standard internal class, no allowance (para 5.5.2); ~ inch 2B.
Mates
M18x2.5-6H takes a thread of the same standard at the same size and pitch. The standard mating external thread is class 6g. 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 M18x2.5-6Hevery column of the workbook row
| Designation | M18x2.5-6H |
|---|---|
| Alternate designation (all capitals, para 7.2.3) | M18X2.5-6H INT |
| 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 | Internal |
| Class | 6H |
| 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 | H |
| Tolerance grade, pitch dia | 6 |
| Tolerance position, crest dia | H |
| Tolerance grade, crest dia | 6 |
| Fundamental deviation es / EI, pitch dia (mm) | 0.000 |
| Fundamental deviation es / EI, crest dia (mm) | 0.000 |
| Allowance |es| / EI (mm) | 0.000 |
| Major dia max (mm) | 18.585 |
| Major dia min (mm) | 18.000 |
| Major dia tolerance Td (mm) | 0.585 |
| Major dia status | D min toleranced; D max for reference (para 9.3.1(d), Table 15 note 2) |
| Basic pitch dia D2, d2 (mm) | 16.376 |
| Pitch dia max (mm) | 16.600 |
| Pitch dia min (mm) | 16.376 |
| PD tolerance Td2 / TD2 (mm) | 0.224 |
| Basic minor dia D1, d1 (mm) | 15.294 |
| Minor dia max (mm) | 15.744 |
| Minor dia min (mm) | 15.294 |
| Minor dia tolerance TD1 (mm) | 0.450 |
| Minor dia status | Toleranced (D1 min, D1 max) |
| Upper deviation, pitch dia es / ES (mm) | 0.224 |
| Lower deviation, pitch dia ei / EI (mm) | 0.000 |
| Upper deviation, crest dia (mm) | 0.450 |
| Lower deviation, crest dia (mm) | 0.000 |
| 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) | 0.065 |
| Equivalent change in functional dia (mm) | 0.112 |
| 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′ |
| 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) | 0.625000 |
| Root flat (mm) | 0.312500 |
| Sweep cut: start diameter (mm) | 15.294000 |
| Sweep cut: tip diameter (mm) | 18.000000 |
| Sweep cut: depth (mm) | 1.353000 |
| Sweep cut: profile width at start diameter (mm) | 1.874693 |
| Crest flat as modelled (mm) | 0.625307 |
| Check against formulas | Calculated with the standard's formulas; no printed limits for this designation. |
| Notes | Standard internal class, no allowance (para 5.5.2); ~ inch 2B. |
Related tools
- Thread standardsEvery ISO and ASME thread series and size, with limits and drawings.
- Thread Spec CalculatorDiameters, tolerances and geometry for ISO metric and unified threads.
- Thread Fit CheckPick a bolt and a nut and see the clearance or interference.
- Twist Drill SizesASME B94.11M drills: sizes, lengths, shanks and tolerances.
- Thread Designation GuideHow to read M10×1.5, 1/4-20 UNC-2A, NPT and more.
- Thread Notations & TermsSymbols and terms of a thread profile, explained.
ASME B1.13M: Metric Screw Threads: M Profile. Source: workbook data (2026-10-10). Confirm against your licensed copy of the standard before use.