UNF · cutting tap · nominal dimensions
5/8-18 UNF
tap drill size.
For 5/8-18 UNF, the reference drill is 37/64: 0.5781 in (14.684 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 |
|---|---|---|---|---|
| 9/16 | fractional | 9/16 in / 14.287 mm | -0.0156 in | 86.6% |
| 19/32 | fractional | 19/32 in / 15.081 mm | +0.0156 in | 43.3% |
| 35/64 | fractional | 35/64 in / 13.891 mm | -0.0312 in | 108.3% |
| 39/64 | fractional | 39/64 in / 15.478 mm | +0.0312 in | 21.7% |
| 17/32 | fractional | 17/32 in / 13.494 mm | -0.0469 in | 129.9% |
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.