Tap drill size for 6-40
#33
0.113 inches · 2.87 mm · 75% thread engagement
| Tap drill (75% thread) | #33 — 0.113" / 2.87 mm |
| Major diameter | 0.138" / 3.51 mm |
| Drill for a 100% thread | 0.1055" / 2.68 mm |
| Threads per inch | 40 |
| Pitch | 0.025" / 0.64 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:
0.138 − 1.299 ÷ 40 = 0.1055 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:
0.138 − 0.974 ÷ 40 = 0.1136 inches
The nearest drill that actually exists is #33 at 0.113 inches, which is 0.0006 inches under the theoretical figure. That is the number printed on every 6-40 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% | 0.1218" | #31 (0.12") | 3.05 mm |
| 65% | 0.1169" | #32 (0.116") | 2.95 mm |
| 75% — the chart value | 0.1136" | #33 (0.113") | 2.87 mm |
| 85% | 0.1104" | #35 (0.11") | 2.79 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.
6-40 in metric
6-40 has a major diameter of 3.51 mm and a pitch of 0.64 mm. The nearest metric tap is M4 at 4 mm, which is 0.49 mm larger — far enough apart that there is no confusing the two. An M4 bolt will not enter a 6-40 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: #33 is 2.87 mm, and a metric drill index will offer 2.9 mm as the nearest size.
What 6-40 is used for
Fine-pitch #6, used where the thread has to hold in thin material.
The same 0.138" screw is also made as 6-32, the coarse version, which takes a #36 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 6-40 tap?
A #33 drill, which is 0.113 inches or 2.87 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 6-40.
What size hole do I need for a 6-40 thread?
The same #33 hole — 0.113 inches. The number comes from subtracting the thread height from the major diameter: 0.138 − 0.974 ÷ 40 = 0.1136 inches, and #33 is the nearest drill that exists.
What is the 6-40 tap drill size in mm?
2.87 mm. There is no metric drill of exactly that size, so a metric drill index gives you 2.9 mm as the nearest — close enough for 6-40 in most materials, though it changes the thread engagement slightly.
Can I use a #32 instead?
Yes, and in hard material you should. #32 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
- 8-32 tap drill size — #29
- 8-36 tap drill size — #29
- 10-24 tap drill size — #25
- 10-32 tap drill size — #21
- 12-24 tap drill size — #16
- 12-28 tap drill size — #14
- 1/4-20 tap drill size — #7
- 1/4-28 tap drill size — #3
- 5/16-18 tap drill size — F
- 5/16-24 tap drill size — I