UNF · cutting tap · nominal dimensions
1/4-28 UNF
tap drill size.
For 1/4-28 UNF, the reference drill is #3: 0.2130 in (5.410 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 |
|---|---|---|---|---|
| 5.4 mm | metric | 0.2126 in / 5.4 mm | -0.0004 in | 80.6% |
| 5.5 mm | metric | 0.2165 in / 5.5 mm | +0.0035 in | 72.1% |
| #4 | number | 0.2090 in / 5.309 mm | -0.0040 in | 88.4% |
| 5.3 mm | metric | 0.2087 in / 5.3 mm | -0.0043 in | 89.1% |
| 7/32 | fractional | 7/32 in / 5.556 mm | +0.0057 in | 67.4% |
| 5.6 mm | metric | 0.2205 in / 5.6 mm | +0.0075 in | 63.6% |
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 provisional.
Tap-drill row: A familiar chart or D-minus-pitch value that was not closed against a single row in the selected OSG/Guhring public tables. Kept visible for audit, never labeled normative or manufacturer-published.
Open Drill Bit Size Chart source
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.