UNF · cutting tap · nominal dimensions
7/8-14 UNF
tap drill size.
For 7/8-14 UNF, the reference drill is 13/16: 0.8125 in (20.637 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 |
|---|---|---|---|---|
| 53/64 | fractional | 53/64 in / 21.034 mm | +0.0156 in | 50.5% |
| 51/64 | fractional | 51/64 in / 20.241 mm | -0.0156 in | 84.2% |
| 25/32 | fractional | 25/32 in / 19.844 mm | -0.0312 in | 101.0% |
| 27/32 | fractional | 27/32 in / 21.431 mm | +0.0312 in | 33.7% |
| 49/64 | fractional | 49/64 in / 19.447 mm | -0.0469 in | 117.9% |
| 55/64 | fractional | 55/64 in / 21.828 mm | +0.0469 in | 16.8% |
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.