Metric coarse · cutting tap · nominal dimensions
M20 × 2.5
tap drill size.
For M20 × 2.5, the reference drill is 17.5 mm: 0.6890 in (17.500 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 |
|---|---|---|---|---|
| 11/16 | fractional | 11/16 in / 17.462 mm | -0.0015 in | 78.1% |
| 45/64 | fractional | 45/64 in / 17.859 mm | +0.0141 in | 65.9% |
| 43/64 | fractional | 43/64 in / 17.066 mm | -0.0171 in | 90.4% |
| 23/32 | fractional | 23/32 in / 18.256 mm | +0.0298 in | 53.7% |
| 21/32 | fractional | 21/32 in / 16.669 mm | -0.0327 in | 102.6% |
| 47/64 | fractional | 47/64 in / 18.653 mm | +0.0454 in | 41.5% |
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.