Tap drill size for 1-64
#53
0.0595 inches · 1.51 mm · 75% thread engagement
| Tap drill (75% thread) | #53 — 0.0595" / 1.51 mm |
| Major diameter | 0.073" / 1.85 mm |
| Drill for a 100% thread | 0.0527" / 1.34 mm |
| Threads per inch | 64 |
| Pitch | 0.0156" / 0.4 mm |
| Series | UNC (coarse) |
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:
0.073 − 1.299 ÷ 64 = 0.0527 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:
0.073 − 0.974 ÷ 64 = 0.0578 inches
The nearest drill that actually exists is #53 at 0.0595 inches, which is 0.0017 inches over the theoretical figure. That is the number printed on every 1-64 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% | 0.0629" | 1/16" (0.0625") | 1.59 mm |
| 65% | 0.0598" | #53 (0.0595") | 1.51 mm |
| 75% — the chart value | 0.0578" | #53 (0.0595") | 1.51 mm |
| 85% | 0.0557" | #54 (0.055") | 1.4 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.
1-64 in metric
1-64 has a major diameter of 1.85 mm and a pitch of 0.4 mm. The closest metric tap is M2, and it is close: 2 mm across against 1.85, 0.4 mm of pitch against 0.4. Close is the problem. An M2 bolt will start in a 1-64 hole and then jam, because it is 0.15 mm fatter than the hole was cut for. The pitches are near enough identical — 0.4 mm against 0.4 — which is precisely why it goes far enough in to feel right. That is the failure people mistake for a damaged thread when it is simply the wrong standard.
For the drill itself the conversion is straightforward: #53 is 1.51 mm, and a metric drill index will offer 1.5 mm as the nearest size.
What 1-64 is used for
Small electronics and hinge screws.
The same 0.073" screw is also made as 1-72, the fine version, which takes a #53 drill instead. Coarse threads tap faster, tolerate damage better and hold better in soft material.
Frequently asked questions
What size drill bit do I need for a 1-64 tap?
A #53 drill, which is 0.0595 inches or 1.51 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 1-64.
What size hole do I need for a 1-64 thread?
The same #53 hole — 0.0595 inches. The number comes from subtracting the thread height from the major diameter: 0.073 − 0.974 ÷ 64 = 0.0578 inches, and #53 is the nearest drill that exists.
What is the 1-64 tap drill size in mm?
1.51 mm. There is no metric drill of exactly that size, so a metric drill index gives you 1.5 mm as the nearest — close enough for 1-64 in most materials, though it changes the thread engagement slightly.
Can I use a #53 instead?
Yes, and in hard material you should. #53 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
- 1-72 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
- 6-32 tap drill size — #36