3/8-16 tap drill size

Drill a 5/16" hole (0.3125" / 7.94 mm) for a 3/8-16 UNC tap. That leaves about 77% thread, the standard for general-purpose tapping.

Tap drill
5/16"
0.3125" · 7.94 mm · 77% thread
Metric drill
8.0 mm 74% thread
Clearance (close)
W 0.3860"
Helicoil (STI) drill
25/64"
3/8-16 tap drill: 5/16" (0.3125 in / 7.94 mm), 77% thread

3/8-16 thread specs

SeriesUnified coarse (UNC)
Major diameter0.3750" (9.52 mm)
Pitch16 TPI (1.59 mm)
Basic minor diameter0.3073" (7.81 mm)
Common internal class2B

Every drill that works for 3/8-16

Drills from all four sets that give 50–85% thread, largest first. Lower percentages tap easier; see how much thread you need.

DrillThreadinmmSetGood for
Q53%0.33208.43Letterdeep holes only
8.4 mm55%0.33078.40Metricdeep holes only
21/64"58%0.32818.33Fractionaldeep holes only
8.3 mm59%0.32688.30Metricdeep holes only
P64%0.32308.20Letterhard or tough metals
8.2 mm64%0.32288.20Metrichard or tough metals
8.1 mm69%0.31898.10Metrichard or tough metals
O73%0.31608.03Lettergeneral use
8.0 mm74%0.31508.00Metricgeneral use
5/16"standard77%0.31257.94Fractionalgeneral use
7.9 mm79%0.31107.90Metricgeneral use
7.8 mm84%0.30717.80Metricmore torque, more broken taps

3/8-16 clearance hole

The hole a 3/8-16 screw passes through. Close fit locates parts; free fit is forgiving.

FitDrillinmm
Close fitW0.38609.80
Free fitX0.397010.08

3/8-16 counterbore and countersink

For a socket head cap screw to sit flush, and for a flat head socket screw.

Socket head counterboreDiameterDepth
AMES5/8" 0.6250"0.3750"

Flat head socket screw: countersink at 82° (inch screws, ASME B18.3) to 0.7810" (19.84 mm) across for a flush head. Metric flat heads use 90°, so don't mix countersinks.

3/8-16 holes in a 3D print

Printed holes usually come out undersize. Model the hole at the drill size plus what your printer loses, then tap or drill to size. Why printed holes shrink.

Print a test hole, measure it with calipers, enter the difference. 0 models the nominal size.

Hole to tap
7.94 mm
Clearance hole
9.80 mm

3/8-16 thread repair (Helicoil)

Stripped 3/8-16 thread? Drill out with the STI drill, tap with a 3/8-16 UNC STI tap, and fit a wire thread insert. The drill in your repair kit’s instructions wins over any chart.

ChartSTI drillinmm
AMES25/64"0.39069.92
MISUMI25/64"0.39069.92

Related sizes

5/16-18next size down1/2-13next size up3/8-24fine threadM10closest metricFull chartUNC / UNF

Frequently asked questions

What size drill do I use for a 3/8-16 tap?
Use a 5/16" (0.3125 in / 7.94 mm) drill. It leaves about 77% thread, the standard for general-purpose tapping.
What metric drill can I use for a 3/8-16 tap?
8.0 mm (0.3150 in / 8.00 mm) is the closest, giving about 74% thread. Anything from about 65% to 80% thread taps well in most materials.
What size clearance hole for a 3/8-16 screw?
W (0.3860") for a close fit, X (0.3970") for a free fit.
What is the metric equivalent of 3/8-16?
M10 is the closest size (10.00 mm vs 9.52 mm major diameter). The pitches differ, so the screws and tapped holes are not interchangeable.

Sources

Spotted a wrong number? Email contact@clove3d.com and it gets fixed.

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% thread = (major − drill) ÷ (1.299 × pitch) × 100. The standard drill comes from published charts and is checked against this formula on every build.