Drill Bit Size Chart
Tools
Data 2026.07.13-audit.1
Checking connection

UNC · cutting tap · nominal dimensions

7/8-9 UNC
tap drill size.

For 7/8-9 UNC, the reference drill is 49/64: 0.7656 in (19.447 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.

Reference drill Provisional
Calibrated relative diameter · 1 in reference
Provisional reference — row-level source not closed
49/64
0.7656 in · 19.447 mm
Calculates to75.8%theoretical full thread
70% target0.7740 in19.659 mm
75% target0.7667 in19.475 mm
Where this drill sits from 0 to 1 inch
49/64 · 19.447 mm on a zero to 25.4 millimetre scale. Neighbor marks: 3/4, 25/32.
0 25.4 mm
Check My Bit Box Print chart

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.

Axial section of 7/8-9 UNC Schematic 60-degree internal and external thread profiles with the common chart drill line and basic internal minor-diameter reference. Differences are exaggerated for explanation. BASIC INTERNAL MINOR-DIAMETER REFERENCE · 83.3% CHART DRILL Ø 0.7656 IN / 19.447 MM PITCH P = 2.822 MM 60° INTERNAL THREAD / WORKPIECE MATING EXTERNAL THREAD — BASIC PROFILE SCHEMATIC · PROFILE DIFFERENCE EXAGGERATED 75.8% THEORETICAL FULL THREAD
The 83.3% line is a basic-profile reference—not a universal interference switch. Actual fit depends on the internal and external thread profiles, class, tolerances, tap, and real hole.

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.

Nearest nominal bits to 49/64
BitSystemDiameterDeltaCalculated full thread
3/4 fractional 3/4 in / 19.050 mm -0.0156 in 86.6%
25/32 fractional 25/32 in / 19.844 mm +0.0156 in 65.0%
47/64 fractional 47/64 in / 18.653 mm -0.0312 in 97.4%
51/64 fractional 51/64 in / 20.241 mm +0.0312 in 54.1%
23/32 fractional 23/32 in / 18.256 mm -0.0469 in 108.3%
13/16 fractional 13/16 in / 20.637 mm +0.0469 in 43.3%
Optional spatial explanation SCHEMATIC · NOT TO SCALE

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.

Nearby UNC threads

3/4-10 UNC1-8 UNC5/8-11 UNC9/16-12 UNC