3/8-24 tap drill size

Drill a Q hole (0.3320" / 8.43 mm) for a 3/8-24 UNF tap. That leaves about 79% thread, the standard for general-purpose tapping.

Tap drill
Q
0.3320" · 8.43 mm · 79% thread
Metric drill
8.5 mm 75% thread
Clearance (close)
W 0.3860"
Helicoil (STI) drill
W
3/8-24 tap drill: Q (0.3320 in / 8.43 mm), 79% thread

3/8-24 thread specs

SeriesUnified fine (UNF)
Major diameter0.3750" (9.52 mm)
Pitch24 TPI (1.06 mm)
Basic minor diameter0.3299" (8.38 mm)
Common internal class2B

Every drill that works for 3/8-24

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

DrillThreadinmmSetGood for
8.8 mm53%0.34658.80Metricdeep holes only
11/32"58%0.34388.73Fractionaldeep holes only
8.7 mm60%0.34258.70Metrichard or tough metals
R67%0.33908.61Letterhard or tough metals
8.6 mm67%0.33868.60Metrichard or tough metals
8.5 mm75%0.33468.50Metricgeneral use
Qstandard79%0.33208.43Lettergeneral use
8.4 mm82%0.33078.40Metricmore torque, more broken taps

3/8-24 clearance hole

The hole a 3/8-24 screw passes through. Close fit locates parts; free fit is forgiving.

FitDrillinmm
Close fitW0.38609.80
Free fitX0.397010.08

3/8-24 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.

3/8-24 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
8.43 mm
Clearance hole
9.80 mm

3/8-24 thread repair (Helicoil)

Stripped 3/8-24 thread? Drill out with the STI drill, tap with a 3/8-24 UNF STI tap, and fit a wire thread insert. The drill in your repair kit’s instructions wins over any chart.

ChartSTI drillinmm
AMESW0.38609.80
MISUMI25/64"0.39069.92

Related sizes

1/4-28next size down1/2-20next size up3/8-16coarse threadM10closest metricFull chartUNC / UNF

Frequently asked questions

What size drill do I use for a 3/8-24 tap?
Use a Q (0.3320 in / 8.43 mm) drill. It leaves about 79% thread, the standard for general-purpose tapping.
What metric drill can I use for a 3/8-24 tap?
8.5 mm (0.3346 in / 8.50 mm) is the closest, giving about 75% thread. Anything from about 65% to 80% thread taps well in most materials.
What size clearance hole for a 3/8-24 screw?
W (0.3860") for a close fit, X (0.3970") for a free fit.
What is the metric equivalent of 3/8-24?
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.