UNC · cutting tap · nominal dimensions
1/2-13 UNC
tap drill size.
For 1/2-13 UNC, the reference drill is 27/64: 0.4219 in (10.716 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 |
|---|---|---|---|---|
| 10.7 mm | metric | 0.4213 in / 10.7 mm | -0.0006 in | 78.8% |
| 10.8 mm | metric | 0.4252 in / 10.8 mm | +0.0033 in | 74.9% |
| 10.6 mm | metric | 0.4173 in / 10.6 mm | -0.0046 in | 82.7% |
| 10.9 mm | metric | 0.4291 in / 10.9 mm | +0.0073 in | 70.9% |
| 10.5 mm | metric | 0.4134 in / 10.5 mm | -0.0085 in | 86.7% |
| Z | letter | 0.4130 in / 10.490 mm | -0.0089 in | 87.1% |
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.