M18 x 1.5 thread pitch and dimensions
M18 x 1.5 is an M18 fine thread with a 1.5 mm pitch (ISO 261), about 16.9 threads per inch. Its basic pitch diameter is 17.026 mm and its minor diameter is 16.376 mm in the nut and 16.160 mm on the bolt (ISO 724 / ISO 898-1).
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| Dimension | Formula | mm | inch |
|---|---|---|---|
| Major diameter d | nominal | 18.000 | 0.7087 |
| Pitch P | ISO 261 | 1.500 | 0.0591 |
| Pitch diameter d2 | d − 0.649519 P | 17.026 | 0.6703 |
| Minor diameter D1 (nut) | d − 1.082532 P | 16.376 | 0.6447 |
| Minor diameter d3 (bolt) | d − 1.226869 P | 16.160 | 0.6362 |
| Fundamental triangle H | 0.866025 P | 1.299 | 0.0511 |
| Thread overlap H1 | 0.541266 P | 0.812 | 0.0320 |
| Thread | Pitch | Series | Pitch Ø d2 | Stress area As | Tap drill |
|---|---|---|---|---|---|
| M18 x 2.5 | 2.5 mm | coarse | 16.376 mm | 192 mm² | 15.5 mm |
| M18 x 1.5 (this page) | 1.5 mm | fine | 17.026 mm | 216 mm² | 16.5 mm |
| Thread | Pitch | Minor Ø nut D1 | Stress area As | Hex AF |
|---|---|---|---|---|
| M14 x 1.5 | 1.5 mm | 12.376 mm | 125 mm² | 21.0 mm |
| M16 x 1.5 | 1.5 mm | 14.376 mm | 167 mm² | 24.0 mm |
| M18 x 1.5 (this page) | 1.5 mm | 16.376 mm | 216 mm² | 27.0 mm |
| M20 x 1.5 | 1.5 mm | 18.376 mm | 272 mm² | 30.0 mm |
| M24 x 2.0 | 2.0 mm | 21.835 mm | 384 mm² | 36.0 mm |
Practical notes
Metric threads are named by major diameter and pitch: M18 x 1.5 means a 18 mm nominal diameter with 1.5 mm between crests. Because it is not the coarse pitch, the pitch must always be written, e.g. "M18 x 1.5".
The pitch diameter and minor diameters above are basic (zero-tolerance) sizes. Real nuts and bolts are made to a tolerance class, usually 6H for the nut and 6g for the bolt (ISO 965), so a measured bolt is slightly under these values.
To identify a thread, measure the major diameter with calipers and the pitch with a thread gauge, or count crests over 10 mm. M18 x 1.5 has about 16.9 threads per inch, which is close to, but not the same as, any Unified thread; an inch nut will not run on it.
Tensile stress area As is what bolt strength is calculated on (proof load = As × proof stress for the property class in ISO 898-1). For M18 x 1.5 it is 216 mm².
FAQ
What is the pitch of an M18 x 1.5 thread?
M18 x 1.5 has a 1.5 mm pitch, a fine pitch in ISO 261. The coarse pitch for M18 is 2.5 mm.
What is the minor diameter of M18 x 1.5?
The basic minor diameter is 16.376 mm for the internal thread (nut, D1 = d − 1.082532 P) and 16.160 mm for the external thread (bolt, d3 = d − 1.226869 P).
What is the tensile stress area of M18 x 1.5?
As = π/4 × ((d2 + d3)/2)² = 216 mm² for M18 x 1.5 (ISO 898-1).
What tap drill does M18 x 1.5 need?
16.5 mm for a cut tap (major diameter minus pitch, ISO 2306), which gives about 77% thread.
What spanner size fits an M18 x 1.5 bolt?
ISO 4032 nuts and ISO 4014/4017 bolts in M18 x 1.5 are 27.0 mm across flats. Older DIN 934/933 parts can differ at M10, M12, M14 and M22.
Sources: ISO 261 — ISO general purpose metric screw threads, general plan (pitch series) · ISO 724 — ISO general purpose metric screw threads, basic dimensions · ISO 898-1 — mechanical properties of fasteners (minor diameter d3, tensile stress area As) · ISO 4032 / ISO 4014 — hexagon nuts and bolts (width across flats)
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