Metric coarse · 2.5 mm pitch
M18×2.5 thread
Every dimension of an M18×2.5 thread as the configurator applies it: major and minor diameter, pitch, the tap drill, the thread-relief undercut, and the standard drills either side.
M18×2.5 is a metric coarse thread: 18 mm nominal major diameter and a 2.5 mm pitch, the distance from one thread crest to the next. That pitch is 10.2 threads per inch, which is how an inch-trained shop reads it; the major diameter is 0.7087 in. It is a 60° V-form thread.
The major diameter, 18.000 mm, is the outside of the external thread and the diameter a shaft end is turned to before the thread is cut. The minor diameter, 15.294 mm, is the root: the thread is 1.353 mm deep per side between the two.
The chart tap drill is 15.5mm (0.6102 in), which leaves about 77 percent of a full thread. No depth-dependent hole window is seeded for metric sizes, so the configurator drills an M18×2.5 tapped hole at 15.5mm at every depth and the drawing callout names that drill.
M18×2.5 thread dimensions
| Dimension | Value | Unit |
|---|---|---|
| Major diameter (nominal) | 18.000 | mm |
| Minor diameter | 15.294 | mm |
| Thread depth, major to minor, per side | 1.353 | mm |
| Pitch | 2.500 | mm |
| Threads per inch (equivalent) | 10.2 | TPI |
| Major diameter (inch equivalent) | 0.7087 | in |
| Minor diameter (inch equivalent) | 0.6021 | in |
| Pitch (inch equivalent) | 0.0984 | in |
| Chart tap drill | 15.5mm | metric drill |
| Chart tap drill diameter | 15.500 | mm |
| Chart tap drill diameter (inch equivalent) | 0.6102 | in |
| Percent of thread at the chart tap drill | 77 | % |
| Thread-relief undercut width (3 × pitch) | 7.500 | mm |
| Thread-relief undercut diameter (reference) | 14.500 | mm |
| Thread-relief undercut diameter the configurator derives | 14.913 | mm |
Millimetres, with inch equivalents at the resolution the configurator holds them. M18×2.5 is a 60° V-form thread. The undercut is the thread-relief groove cut ahead of a shoulder for a single-point-cut external thread, so the tool has somewhere to stop: 3 × pitch wide (DIN 76 Form A) with its root 0.0150 in below the minor diameter, which is how the configurator derives it when a thread ends in an undercut. The reference undercut diameter is the series' tabulated value; the configurator cuts the derived one.
Standard drills around the M18×2.5 tap drill
| Drill | Family | Ø (in) | Ø (mm) | Percent of thread |
|---|---|---|---|---|
| 14.5mm (under) | metric | 0.5709 | 14.500 | 108 |
| 15.0mm (under) | metric | 0.5906 | 15.000 | 92 |
| 15.5mm (chart tap drill) | metric | 0.6102 | 15.500 | 77 |
| 16.0mm (over) | metric | 0.6299 | 16.000 | 62 |
| 16.5mm (over) | metric | 0.6496 | 16.500 | 46 |
The two metric drills just under and just over the chart tap drill, with the chart drill between them. Percent of thread is (major diameter − drill diameter) ÷ (1.299 × pitch), the figure the configurator reports on the callout; a smaller drill leaves more thread and loads the tap harder.
Part of Thread and tap-drill reference. Other pages in the series: 1/4-20 UNC · 5/16-18 UNC · 3/8-16 UNC · 7/16-14 UNC · 1/2-13 UNC · 9/16-12 UNC · 5/8-11 UNC · 3/4-10 UNC · 7/8-9 UNC · 1-8 UNC · 1-1/8-7 UNC · 1-1/4-7 UNC · 1-3/8-6 UNC · 1-1/2-6 UNC · 1/4-28 UNF · 5/16-24 UNF · 3/8-24 UNF · 7/16-20 UNF · 1/2-20 UNF · 9/16-18 UNF · 5/8-18 UNF · 3/4-16 UNF · 7/8-14 UNF · 1-12 UNF · M6×1 · M8×1.25 · M10×1.5 · M12×1.75 · M14×2 · M16×2 · M20×2.5 · M22×2.5 · M24×3.
Thread relief undercut for M18×2.5
An M18×2.5 external thread that runs up to a shoulder ends in a relief groove 7.500 mm wide, three times the pitch, with its root at 14.913 mm, 0.381 mm below the minor diameter. Without it a full-depth single-point thread has nowhere for the tool to stop. Choose an undercut termination on the thread and the configurator derives that groove and prices it as the extra grooving pass it is; choose a runout and the thread simply fades with no groove.
Frequently Asked Questions
These references are for specifying a part; every value here is one the configurator applies when you build one. Numbers are representative of the standard; check them against your part and its mating components before ordering. See all guides on the guides page.