BenchReference

7/8″-9 UNC Thread

Recommended tap drill 49/64″
Tap drill Ø 0.7656″ / 19.447 mm
Major Ø 0.8750″ / 22.225 mm
Threads per inch 9

The chart recommendation gives approximately 75.8% thread engagement — the conventional target for average materials. It is not the only valid choice; see the options below.

Specification

Designation7/8″-9 UNC
Thread familyUnified National Coarse (UNC)
Nominal size7/8
Basic major diameter 0.8750″ / 22.225 mm
Threads per inch / pitch9 TPI (2.822 mm pitch)
Recommended tap drill 49/64″ (0.7656″ / 19.447 mm)
P = 0.1111″ (9 TPI) 60° major Ø 0.8750″ 60° thread form (UN / ISO) — schematic

Tap drill options by thread engagement

Any of these standard drills produces a usable engagement for 7/8″-9 UNC; smaller drills give stronger threads but higher tapping torque. Percentages use the conventional approximation (60° thread form).

49/64″ ★ 75.8% 20 mm 60.7% shaded = share of the full thread height the tap forms · accent line = drilled hole wall
★ = chart recommendation. A smaller drill leaves more of the thread height for the tap to form.
Drill Decimal in mm Approx. engagement
49/64″ — chart recommendation 0.7656 19.447 75.8%
19.5 mm 0.7677 19.5 74.3%
25/32″ 0.7813 19.844 65%
20 mm 0.7874 20 60.7%

Nearest drills to the tap drill in other systems

Drill Size vs 49/64″ Resulting engagement
19.5 mm 0.7677″ / 19.5 mm +0.053 mm 74.3%

Clearance holes

For a bolt or screw of this size to pass through freely. Recommended drills per ASME B18.2.8: close, normal, and loose fits.

Tapped hole Clearance hole 49/64" tap drill threads cut into the material 15/16" (normal fit) the tinted gap lets the bolt pass freely
Schematic — proportions exaggerated for clarity; use the table values.
Fit Drill / Ø Decimal in mm
close 29/32″ 0.9063 23.019
normal 15/16″ 0.9375 23.813
loose 1-1/32″ 1.0313 26.194

Hex bolt & nut dimensions

For a standard hex cap screw (ASME B18.2.1) and finished hex nut (ASME B18.2.2) in this size.

Wrench / socket size (bolt head) 1-5/16″
Head height 35/64″ (0.5469″)
Nut thickness 3/4″ (0.7500″)
Compatible nut 7/8-9 hex nut
Head — top view Bolt — side view Nut — side view width across flats = 1-5/16″ wrench head height 35/64″ thickness 3/4″

Basic (nominal) dimensions. See the wrench & socket size charts for all sizes and the metric ↔ SAE socket comparison.

Related threads

Data provenance. ASME B1.1, Unified Inch Screw Threads (ASME). Machinery’s Handbook — tap drill charts (Industrial Press). ASME B18.2.8, Clearance Holes for Bolts, Screws, and Studs (ASME). ASME B18.2.1, Hex Cap Screws (head dimensions) (ASME). ASME B18.2.2, Finished Hex Nuts (ASME). Engagement percentages use the conventional approximation (engagement = (major − drill) ÷ (1.29904 × pitch)) and are guidance, not a tolerance specification. See all data sources.