BenchReference

#5-40 UNC Thread

Recommended tap drill #38
Tap drill Ø 0.1015″ / 2.578 mm
Major Ø 0.1250″ / 3.175 mm
Threads per inch 40

The chart recommendation gives approximately 72.4% thread engagement — the conventional target for average materials. It is not the only valid choice; see the options below.

Specification

Designation#5-40 UNC
Thread familyUnified National Coarse (UNC)
Nominal size#5
Basic major diameter 0.1250″ / 3.175 mm
Threads per inch / pitch40 TPI (0.635 mm pitch)
Recommended tap drill #38 (0.1015″ / 2.578 mm)
P = 0.0250″ (40 TPI) 60° major Ø 0.1250″ 60° thread form (UN / ISO) — schematic

Tap drill options by thread engagement

Any of these standard drills produces a usable engagement for #5-40 UNC; smaller drills give stronger threads but higher tapping torque. Percentages use the conventional approximation (60° thread form).

#40 83.1% #38 ★ 72.4% #37 64.7% 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
#40 0.0980 2.489 83.1%
2.5 mm 0.0984 2.5 81.8%
#39 0.0995 2.527 78.5%
#38 — chart recommendation 0.1015 2.578 72.4%
2.6 mm 0.1024 2.6 69.7%
#37 0.1040 2.642 64.7%

Nearest drills to the tap drill in other systems

Drill Size vs #38 Resulting engagement
2.6 mm 0.1024″ / 2.6 mm +0.022 mm 69.7%

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 #38 tap drill threads cut into the material 5/32" (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 9/64″ 0.1406 3.572
normal 5/32″ 0.1563 3.969
loose 11/64″ 0.1719 4.366

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). Engagement percentages use the conventional approximation (engagement = (major − drill) ÷ (1.29904 × pitch)) and are guidance, not a tolerance specification. See all data sources.