Tap drill size for 0-80
3/64"
0.0469 inches · 1.19 mm · 75% thread engagement
| Tap drill (75% thread) | 3/64" — 0.0469" / 1.19 mm |
| Major diameter | 0.06" / 1.52 mm |
| Drill for a 100% thread | 0.0438" / 1.11 mm |
| Threads per inch | 80 |
| Pitch | 0.0125" / 0.32 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:
0.06 − 1.299 ÷ 80 = 0.0438 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:
0.06 − 0.974 ÷ 80 = 0.0478 inches
The nearest drill that actually exists is 3/64" at 0.0469 inches, which is 0.0009 inches under the theoretical figure. That is the number printed on every 0-80 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% | 0.0519" | #55 (0.052") | 1.32 mm |
| 65% | 0.0494" | #55 (0.052") | 1.32 mm |
| 75% — the chart value | 0.0478" | 3/64" (0.0469") | 1.19 mm |
| 85% | 0.0462" | #56 (0.0465") | 1.18 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.
0-80 in metric
0-80 has a major diameter of 1.52 mm and a pitch of 0.32 mm. The nearest metric tap is M2 at 2 mm, which is 0.48 mm larger — far enough apart that there is no confusing the two. An M2 bolt will not enter a 0-80 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: 3/64" is 1.19 mm, and a metric drill index will offer 1.2 mm as the nearest size.
What 0-80 is used for
Instrument and optical work; the smallest unified thread in general use.
Frequently asked questions
What size drill bit do I need for a 0-80 tap?
A 3/64" drill, which is 0.0469 inches or 1.19 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 0-80.
What size hole do I need for a 0-80 thread?
The same 3/64" hole — 0.0469 inches. The number comes from subtracting the thread height from the major diameter: 0.06 − 0.974 ÷ 80 = 0.0478 inches, and 3/64" is the nearest drill that exists.
What is the 0-80 tap drill size in mm?
1.19 mm. There is no metric drill of exactly that size, so a metric drill index gives you 1.2 mm as the nearest — close enough for 0-80 in most materials, though it changes the thread engagement slightly.
Can I use a #55 instead?
Yes, and in hard material you should. #55 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
- 1-64 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
- 4-48 tap drill size — #42
- 5-40 tap drill size — #38
- 5-44 tap drill size — #37