UNC · cutting tap · nominal dimensions
5/16-18 UNC
tap drill size.
For 5/16-18 UNC, the reference drill is F: 0.2570 in (6.528 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 |
|---|---|---|---|---|
| 6.5 mm | metric | 0.2559 in / 6.5 mm | -0.0011 in | 78.4% |
| 6.6 mm | metric | 0.2598 in / 6.6 mm | +0.0028 in | 73.0% |
| G | letter | 0.2610 in / 6.629 mm | +0.0040 in | 71.4% |
| 6.4 mm | metric | 0.2520 in / 6.4 mm | -0.0050 in | 83.9% |
| 6.7 mm | metric | 0.2638 in / 6.7 mm | +0.0068 in | 67.5% |
| E | letter | 0.2500 in / 6.350 mm | -0.0070 in | 86.6% |
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.