Tap drill size for 7/8-14
13/16"
0.8125 inches · 20.64 mm · 75% thread engagement
| Tap drill (75% thread) | 13/16" — 0.8125" / 20.64 mm |
| Major diameter | 0.875" / 22.23 mm |
| Drill for a 100% thread | 0.7822" / 19.87 mm |
| Threads per inch | 14 |
| Pitch | 0.0714" / 1.81 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:
0.875 − 1.299 ÷ 14 = 0.7822 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:
0.875 − 0.974 ÷ 14 = 0.8054 inches
The nearest drill that actually exists is 13/16" at 0.8125 inches, which is 0.0071 inches over the theoretical figure. That is the number printed on every 7/8-14 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% | 0.8286" | 53/64" (0.8281") | 21.03 mm |
| 65% | 0.8147" | 13/16" (0.8125") | 20.64 mm |
| 75% — the chart value | 0.8054" | 13/16" (0.8125") | 20.64 mm |
| 85% | 0.7961" | 51/64" (0.7969") | 20.24 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.
7/8-14 in metric
7/8-14 has a major diameter of 22.23 mm and a pitch of 1.81 mm. The nearest metric tap is M24 at 24 mm, which is 1.78 mm larger — far enough apart that there is no confusing the two. An M24 bolt will not enter a 7/8-14 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: 13/16" is 20.64 mm, and a metric drill index will offer 20.6 mm as the nearest size.
What 7/8-14 is used for
Fine-pitch large fittings.
The same 0.875" screw is also made as 7/8-9, the coarse version, which takes a 49/64" 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 7/8-14 tap?
A 13/16" drill, which is 0.8125 inches or 20.64 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 7/8-14.
What size hole do I need for a 7/8-14 thread?
The same 13/16" hole — 0.8125 inches. The number comes from subtracting the thread height from the major diameter: 0.875 − 0.974 ÷ 14 = 0.8054 inches, and 13/16" is the nearest drill that exists.
What is the 7/8-14 tap drill size in mm?
20.64 mm. There is no metric drill of exactly that size, so a metric drill index gives you 20.6 mm as the nearest — close enough for 7/8-14 in most materials, though it changes the thread engagement slightly.
Can I use a 13/16" instead?
Yes, and in hard material you should. 13/16" 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
- 1-8 tap drill size — 7/8"
- 1-12 tap drill size — 59/64"
- 0-80 tap drill size — 3/64"
- 1-64 tap drill size — #53
- 1-72 tap drill size — #53
- 2-56 tap drill size — #50
- 2-64 tap drill size — #50
- 3-48 tap drill size — #47
- 3-56 tap drill size — #45
- 4-40 tap drill size — #43