UNC · cutting tap · nominal dimensions
1/4-20 UNC
tap drill size.
For 1/4-20 UNC, the reference drill is #7: 0.2010 in (5.105 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 |
|---|---|---|---|---|
| 5.1 mm | metric | 0.2008 in / 5.1 mm | -0.0002 in | 75.8% |
| #8 | number | 0.1990 in / 5.055 mm | -0.0020 in | 78.5% |
| 13/64 | fractional | 13/64 in / 5.159 mm | +0.0021 in | 72.2% |
| #6 | number | 0.2040 in / 5.182 mm | +0.0030 in | 70.8% |
| 5.2 mm | metric | 0.2047 in / 5.2 mm | +0.0037 in | 69.7% |
| 5 mm | metric | 0.1969 in / 5 mm | -0.0041 in | 81.8% |
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.