UNC · cutting tap · nominal dimensions
3/8-16 UNC
tap drill size.
For 3/8-16 UNC, the reference drill is 5/16: 0.3125 in (7.938 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 |
|---|---|---|---|---|
| 7.9 mm | metric | 0.3110 in / 7.9 mm | -0.0015 in | 78.8% |
| 8 mm | metric | 0.3150 in / 8 mm | +0.0025 in | 73.9% |
| O | letter | 0.3160 in / 8.026 mm | +0.0035 in | 72.7% |
| 7.8 mm | metric | 0.3071 in / 7.8 mm | -0.0054 in | 83.6% |
| 8.1 mm | metric | 0.3189 in / 8.1 mm | +0.0064 in | 69.1% |
| 7.7 mm | metric | 0.3031 in / 7.7 mm | -0.0094 in | 88.5% |
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.