Metric coarse · cutting tap · nominal dimensions
M18 × 2.5
tap drill size.
For M18 × 2.5, the reference drill is 15.5 mm: 0.6102 in (15.500 mm).
The reference value is kept separate from the 70% and 75% calculated targets. Actual fit still depends on thread class, tool, material, runout, and the hole you really make.
Detail A · basic 60° profile
What the percentage describes.
“Theoretical full thread %” is radial thread height from nominal geometry. It is not axial engagement length and not a fit-class acceptance measurement.
Real drill alternatives
Neighboring sizes.
These are diameter neighbors, not silent recommendations. A positive delta makes a larger hole and a lower nominal theoretical percentage; a negative delta does the opposite.
| Bit | System | Diameter | Delta | Calculated full thread |
|---|---|---|---|---|
| 39/64 | fractional | 39/64 in / 15.478 mm | -0.0009 in | 77.7% |
| 5/8 | fractional | 5/8 in / 15.875 mm | +0.0148 in | 65.4% |
| 19/32 | fractional | 19/32 in / 15.081 mm | -0.0165 in | 89.9% |
| 41/64 | fractional | 41/64 in / 16.272 mm | +0.0304 in | 53.2% |
| 37/64 | fractional | 37/64 in / 14.684 mm | -0.0321 in | 102.1% |
| 21/32 | fractional | 21/32 in / 16.669 mm | +0.0460 in | 41.0% |
Why 3D is useful here
A thread is a helix, not a row of triangles.
Load a draggable cutaway to see the continuous internal thread. Exact diameter and profile comparisons remain in the 2D drawing above.
Evidence and limits
Why this row says cross-checked.
Tap-drill row: Rows tagged with this source were matched to a named 65% or 75% cutting-tap column. The discrete drill can calculate to a slightly different percentage from nominal dimensions.
60° geometry: Public primary reference for Unified and metric 60-degree thread geometry. The percentage-thread equation is a theoretical basic-profile calculation, not a fit or torque guarantee.
Open National Institute of Standards and Technology geometry source
The repository also records 11 source records and a dataset version on every page.