Tap drill size for 7/16-20
25/64"
0.3906 inches · 9.92 mm · 75% thread engagement
| Tap drill (75% thread) | 25/64" — 0.3906" / 9.92 mm |
| Major diameter | 0.4375" / 11.11 mm |
| Drill for a 100% thread | 0.3726" / 9.46 mm |
| Threads per inch | 20 |
| Pitch | 0.05" / 1.27 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:
0.4375 − 1.299 ÷ 20 = 0.3726 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:
0.4375 − 0.974 ÷ 20 = 0.3888 inches
The nearest drill that actually exists is 25/64" at 0.3906 inches, which is 0.0018 inches over the theoretical figure. That is the number printed on every 7/16-20 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% | 0.405" | Y (0.404") | 10.26 mm |
| 65% | 0.3953" | X (0.397") | 10.08 mm |
| 75% — the chart value | 0.3888" | 25/64" (0.3906") | 9.92 mm |
| 85% | 0.3823" | W (0.386") | 9.8 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.
7/16-20 in metric
7/16-20 has a major diameter of 11.11 mm and a pitch of 1.27 mm. The nearest metric tap is M12 at 12 mm, which is 0.89 mm larger — far enough apart that there is no confusing the two. An M12 bolt will not enter a 7/16-20 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: 25/64" is 9.92 mm, and a metric drill index will offer 9.9 mm as the nearest size.
What 7/16-20 is used for
Fine-pitch structural and automotive work.
The same 0.4375" screw is also made as 7/16-14, the coarse version, which takes a U 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 7/16-20 tap?
A 25/64" drill, which is 0.3906 inches or 9.92 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 7/16-20.
What size hole do I need for a 7/16-20 thread?
The same 25/64" hole — 0.3906 inches. The number comes from subtracting the thread height from the major diameter: 0.4375 − 0.974 ÷ 20 = 0.3888 inches, and 25/64" is the nearest drill that exists.
What is the 7/16-20 tap drill size in mm?
9.92 mm. There is no metric drill of exactly that size, so a metric drill index gives you 9.9 mm as the nearest — close enough for 7/16-20 in most materials, though it changes the thread engagement slightly.
Can I use a X instead?
Yes, and in hard material you should. X 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
- 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"
- 7/8-9 tap drill size — 49/64"
- 7/8-14 tap drill size — 13/16"