MJ18 × 2.5 MJ profile External Thread Specifications
MJ18 x 2.5 - 4h6h
ASMESTD: ASME B1.21MSOURCE: Table 7 · p. 24TYPE: External (bolt/screw)CLASS: 4h6hSERIES: Coarse pitchAerospace class, no allowance (approx. inch Class 3A)1st choice (Col. 1)
Major diameter
18.000 – 17.665
max – min
Pitch diameter
16.376 – 16.270
max – min
Root diameter d3
15.113 – 14.856
max – min
Root radius R
0.375 – 0.451
min – max (0.15011P – 0.18042P)
Thread profile
MJ18 x 2.5 - 4h6h: solid line = maximum material profile, dashed = minimum material, hatched = tolerance zone, grey dashed = basic profile. Drawn to scale; values in mm, diameters shortened.
Limits of size, class 4h6h
| Dimension | Basic | Max | Min | Tolerance |
|---|---|---|---|---|
| Major diameter dOver the crests; Td grade 6 (Table 9) | 18.000 | 18.000 | 17.665 | 0.335 |
| Pitch diameter d2Sets the fit | 16.37620 | 16.376 | 16.270 | 0.106 |
| Root (minor) diameter d3Bottom of the root radius (Figs. 3, 4); form diameter d1 max = d1 basic | 15.56430 | 15.113 | 14.856 | 0.257 |
| Root radius RContinuous, tangent to the flanks (Table 1, para. 2.4.2) | 0.451 | 0.375 | ||
| Allowance esSpace for coating | 0.000 |
Dimensions in millimetres. Values are those of the standard (ASME B1.21M); calculated cells follow the standard’s formulas. 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
Lead, flank angle and length of engagement
- Allowable lead variation ±δP
- 0.0306 mmBetween any two points within the GO gage length
- Equivalent change in functional diameter δd2
- 0.0530 mmHalf the pitch diameter tolerance
- Allowable half-angle variation ±δα
- 0° 50′ Each 30° flank
- Cumulative form variation
- 0.0530 mmWhen specified for H/h: 0.5 × PD tolerance (para. 4.4.1)
- Runout of major/minor to pitch cylinder FIM
- 0.106 mmMax FIM when measured (para. 4.4.3)
- Length of engagement, normal upper limit LE
- 27.000 mmTable 12 / 13 allow the tolerance for this range
Lead source: Table 12, p. 45; half-angle source: Table 13, p. 50
MJ root radius and minor diameter paras. 1.1, 2.4.2, Fig. 4
- Root tangency depth 0.5625H
- 1.21785 mmRadius tangent to the flanks at not less than this depth below the basic crest (para. 2.4.2)
- Form diameter d1 max
- 15.56430 mmd1 max = d1 basic
- Root radius implied by d3 max and d2 max R
- 0.451 mmBetween R min 0.15011P and R max 0.18042P
- Coating allowance, pitch diameter
- 0.032 mm
- Coating allowance, major and minor diameter
- 0.019 mm
Notes
- MJ external root: a continuous radius, never less than R min 0.375 mm (0.15011P), tangent to the flanks at not less than 0.5625H below the basic crest; root diameter d3 must lie between 14.856 and 15.113 mm (para. 2.4.2, Fig. 4). Two tangent arcs and a flat are allowed.
Mates
MJ18 x 2.5 - 4h6h screws into a thread of the same standard at the same size and pitch. The standard mating internal thread is class 4H5H. 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 MJ18 x 2.5 - 4h6hevery column of the workbook row
| Designation | MJ18 x 2.5 - 4h6h |
|---|---|
| Alternate designation | MJ18x2.5-4h6h |
| Pitch (mm) | 2.50 |
| Thread form | MJ |
| Series (Table 2) | Coarse |
| Series name | Coarse pitch |
| External / internal | External |
| Class | 4h6h |
| Nominal size (mm) | 18 |
| Pitch P (mm) | 2.50 |
| Basic major dia D, d (mm) | 18.000 |
| Size category | 1st choice (Col. 1) |
| Source table | Table 7 |
| Printed page | 24 |
| Major dia max (mm) | 18.000 |
| Major dia min (mm) | 17.665 |
| Major dia tolerance (mm) | 0.335 |
| PD tolerance LE basis | 1D |
| LE for PD tolerance (mm) | 18.000 |
| Allowance es / EI (mm) | 0.000 |
| Major dia status | Toleranced (Td grade 6, Table 9) |
| Basic pitch dia D2, d2 (mm) | 16.37620 |
| Pitch dia max (mm) | 16.376 |
| Pitch dia min (mm) | 16.270 |
| PD tolerance (mm) | 0.106 |
| Basic minor dia D1, d1 (mm) | 15.56430 |
| Minor dia max (mm) | 15.113 |
| Minor dia min (mm) | 14.856 |
| Minor dia tolerance (mm) | 0.257 |
| Minor dia status | Root dia d3 of the rounded root (Fig. 3, 4): toleranced |
| Root radius max (mm) | 0.451 |
| Root radius min (mm) | 0.375 |
| Lead variation ± (mm) | 0.0306 |
| Equivalent change in functional dia (mm) | 0.0530 |
| Lead values source | Table 12, p. 45 |
| Half-angle variation ± | 0° 50′ |
| Half-angle source | Table 13, p. 50 |
| Lead angle at basic PD (deg) | 2.7820 |
| Lead angle (deg min) | 2° 47′ |
| Tensile stress area As (mm²) | not given in the standard |
| Thread angle (deg) | 60 |
| Flank angle (deg) | 30 |
| Sharp V height H (mm) | 2.16506 |
| Thread height h (mm) | 1.44338 |
| Crest flat (mm) | 0.31250 |
| Root flat (mm) | none: root radius |
| Root radius for model (mm) | 0.45105 |
| Sweep cut: start diameter (mm) | 18.000 |
| Sweep cut: tip diameter (mm) | 15.113 |
| Sweep cut: depth (mm) | 1.44350 |
| Sweep cut: profile width at start diameter (mm) | 2.18750 |
| Crest flat as modelled (mm) | 0.31250 |
| Root radius tangency depth 0.5625H (mm) | 1.21785 |
| Form diameter d1 max = d1 bsc (mm) | 15.56430 |
| Root radius implied by d3 max and d2 max (mm) | 0.451 |
| Normal LE upper limit (mm) | 27.000 |
| PD tolerance, LE over 1.5D/15P to 3D/30P (×1.25) (mm) | 0.133 |
| PD tolerance, LE over 3D/30P (×1.5) (mm) | 0.159 |
| Cumulative form variation 0.5×PD tol (mm) | 0.0530 |
| Runout max FIM = PD tol (mm) | 0.106 |
| Coating allowance, pitch dia (mm) | 0.032 |
| Coating allowance, major and minor dia (mm) | 0.019 |
| Listed in ISO 261 | Yes |
| Class use | Aerospace class, no allowance (approx. inch Class 3A) |
| Check against formulas | OK |
| Notes | MJ external root: a continuous radius, never less than R min 0.375 mm (0.15011P), tangent to the flanks at not less than 0.5625H below the basic crest; root diameter d3 must lie between 14.856 and 15.113 mm (para. 2.4.2, Fig. 4). Two tangent arcs and a flat are allowed. |
| PD tolerance grade | 4 |
| Tolerance position | h |
| Allowable taper (mm), para. 4.4.4 | 0.106 |
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ASME B1.21M: Metric Screw Threads: MJ Profile. Source: workbook data (2026-10-11). Confirm against your licensed copy of the standard before use.