UNC · cutting tap · nominal dimensions
#2-56 UNC
tap drill size.
For #2-56 UNC, the reference drill is #50: 0.0700 in (1.778 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 |
|---|---|---|---|---|
| 1.8 mm | metric | 0.0709 in / 1.8 mm | +0.0009 in | 65.2% |
| #49 | number | 0.0730 in / 1.854 mm | +0.0030 in | 56.0% |
| #51 | number | 0.0670 in / 1.702 mm | -0.0030 in | 81.9% |
| 1.7 mm | metric | 0.0669 in / 1.7 mm | -0.0031 in | 82.2% |
| 1.9 mm | metric | 0.0748 in / 1.9 mm | +0.0048 in | 48.3% |
| #48 | number | 0.0760 in / 1.930 mm | +0.0060 in | 43.1% |
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.