5/16-18 tap drill size

Drill a F hole (0.2570" / 6.53 mm) for a 5/16-18 UNC tap. That leaves about 77% thread, the standard for general-purpose tapping.

Tap drill
F
0.2570" · 6.53 mm · 77% thread
Metric drill
6.6 mm 73% thread
Clearance (close)
P 0.3230"
Helicoil (STI) drill
21/64"
5/16-18 tap drill: F (0.2570 in / 6.53 mm), 77% thread

5/16-18 thread specs

SeriesUnified coarse (UNC)
Major diameter0.3125" (7.94 mm)
Pitch18 TPI (1.41 mm)
Basic minor diameter0.2524" (6.41 mm)
Common internal class2B

Every drill that works for 5/16-18

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

DrillThreadinmmSetGood for
7.0 mm51%0.27567.00Metricdeep holes only
I56%0.27206.91Letterdeep holes only
6.9 mm57%0.27176.90Metricdeep holes only
6.8 mm62%0.26776.80Metrichard or tough metals
H64%0.26606.76Letterhard or tough metals
17/64"65%0.26566.75Fractionalhard or tough metals
6.7 mm68%0.26386.70Metrichard or tough metals
G71%0.26106.63Lettergeneral use
6.6 mm73%0.25986.60Metricgeneral use
Fstandard77%0.25706.53Lettergeneral use
6.5 mm78%0.25596.50Metricgeneral use
6.4 mm84%0.25206.40Metricmore torque, more broken taps

5/16-18 clearance hole

The hole a 5/16-18 screw passes through. Close fit locates parts; free fit is forgiving.

FitDrillinmm
Close fitP0.32308.20
Free fitQ0.33208.43

5/16-18 counterbore and countersink

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

Socket head counterboreDiameterDepth
AMES17/32" 0.5313"0.3120"

Flat head socket screw: countersink at 82° (inch screws, ASME B18.3) to 0.6560" (16.66 mm) across for a flush head. Metric flat heads use 90°, so don't mix countersinks.

5/16-18 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
6.53 mm
Clearance hole
8.20 mm

5/16-18 thread repair (Helicoil)

Stripped 5/16-18 thread? Drill out with the STI drill, tap with a 5/16-18 UNC STI tap, and fit a wire thread insert. The drill in your repair kit’s instructions wins over any chart.

ChartSTI drillinmm
AMES21/64"0.32818.33
MISUMI21/64"0.32818.33

Related sizes

1/4-20next size down3/8-16next size upM8closest metricFull chartUNC / UNF

Frequently asked questions

What size drill do I use for a 5/16-18 tap?
Use a F (0.2570 in / 6.53 mm) drill. It leaves about 77% thread, the standard for general-purpose tapping.
What metric drill can I use for a 5/16-18 tap?
6.6 mm (0.2598 in / 6.60 mm) is the closest, giving about 73% thread. Anything from about 65% to 80% thread taps well in most materials.
What size clearance hole for a 5/16-18 screw?
P (0.3230") for a close fit, Q (0.3320") for a free fit.
What is the metric equivalent of 5/16-18?
M8 is the closest size (8.00 mm vs 7.94 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.