Tap drill size for 3/8-24
Q
0.332 inches · 8.43 mm · 75% thread engagement
| Tap drill (75% thread) | Q — 0.332" / 8.43 mm |
| Major diameter | 0.375" / 9.52 mm |
| Drill for a 100% thread | 0.3209" / 8.15 mm |
| Threads per inch | 24 |
| Pitch | 0.0417" / 1.06 mm |
| Series | UNF (fine) |
Where the number comes from
A tap cuts the thread; the drill leaves the hole it cuts into. Cutting the full thread form would mean drilling the major diameter less the whole height of the thread:
0.375 − 1.299 ÷ 24 = 0.3209 inches
That is the 100% thread, which nobody drills — the tap has to remove every scrap of metal and usually breaks trying. Charts give 75%, so three quarters of the thread height is cut away and the rest is left as clearance:
0.375 − 0.974 ÷ 24 = 0.3344 inches
The nearest drill that actually exists is Q at 0.332 inches, which is 0.0024 inches under the theoretical figure. That is the number printed on every 3/8-24 tap chart, and this is where it comes from.
The same hole at other engagements
Thread engagement is a choice, not a constant. Less engagement means an easier cut and a longer tap life for very little loss of strength, which is why a machinist working stainless will not drill the chart size.
| Engagement | Theoretical | Drill | Millimetres |
|---|---|---|---|
| 50% | 0.3479" | S (0.348") | 8.84 mm |
| 65% | 0.3398" | R (0.339") | 8.61 mm |
| 75% — the chart value | 0.3344" | Q (0.332") | 8.43 mm |
| 85% | 0.329" | 21/64" (0.3281") | 8.33 mm |
A 75% thread is close to the practical ceiling: going from 75% to 100% adds only a few per cent of strength and roughly doubles the torque needed to turn the tap. Going the other way, a 65% thread still holds about 95% of the load a 75% thread does, which is why 65% is the usual choice in anything harder than mild steel.
3/8-24 in metric
3/8-24 has a major diameter of 9.52 mm and a pitch of 1.06 mm. The nearest metric tap is M10 at 10 mm, which is 0.48 mm larger — far enough apart that there is no confusing the two. An M10 bolt will not enter a 3/8-24 hole at all, so if a fastener nearly fits, neither of these is the standard you are looking for.
For the drill itself the conversion is straightforward: Q is 8.43 mm, and a metric drill index will offer 8.4 mm as the nearest size.
What 3/8-24 is used for
Automotive suspension and brake fittings.
The same 0.375" screw is also made as 3/8-16, the coarse version, which takes a 5/16" drill instead. Fine threads hold better in thin material, resist vibration loosening and give finer adjustment.
Frequently asked questions
What size drill bit do I need for a 3/8-24 tap?
A Q drill, which is 0.332 inches or 8.43 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 3/8-24.
What size hole do I need for a 3/8-24 thread?
The same Q hole — 0.332 inches. The number comes from subtracting the thread height from the major diameter: 0.375 − 0.974 ÷ 24 = 0.3344 inches, and Q is the nearest drill that exists.
What is the 3/8-24 tap drill size in mm?
8.43 mm. There is no metric drill of exactly that size, so a metric drill index gives you 8.4 mm as the nearest — close enough for 3/8-24 in most materials, though it changes the thread engagement slightly.
Can I use a R instead?
Yes, and in hard material you should. R gives about 65% thread engagement instead of 75%. That costs very little strength — a 65% thread holds roughly 95% of what a 75% thread does — and it makes the tap markedly easier to turn, which is the trade every machinist makes in stainless or tough alloy.
Other tap sizes
- 7/16-14 tap drill size — U
- 7/16-20 tap drill size — 25/64"
- 1/2-13 tap drill size — 27/64"
- 1/2-20 tap drill size — 29/64"
- 9/16-12 tap drill size — 31/64"
- 9/16-18 tap drill size — 33/64"
- 5/8-11 tap drill size — 17/32"
- 5/8-18 tap drill size — 37/64"
- 3/4-10 tap drill size — 21/32"
- 3/4-16 tap drill size — 11/16"