Tap drill size for 2-56
#50
0.07 inches · 1.78 mm · 75% thread engagement
| Tap drill (75% thread) | #50 — 0.07" / 1.78 mm |
| Major diameter | 0.086" / 2.18 mm |
| Drill for a 100% thread | 0.0628" / 1.6 mm |
| Threads per inch | 56 |
| Pitch | 0.0179" / 0.45 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:
0.086 − 1.299 ÷ 56 = 0.0628 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:
0.086 − 0.974 ÷ 56 = 0.0686 inches
The nearest drill that actually exists is #50 at 0.07 inches, which is 0.0014 inches over the theoretical figure. That is the number printed on every 2-56 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% | 0.0744" | #49 (0.073") | 1.85 mm |
| 65% | 0.0709" | #50 (0.07") | 1.78 mm |
| 75% — the chart value | 0.0686" | #50 (0.07") | 1.78 mm |
| 85% | 0.0663" | #51 (0.067") | 1.7 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.
2-56 in metric
2-56 has a major diameter of 2.18 mm and a pitch of 0.45 mm. The closest metric tap is M2, and it is close: 2 mm across against 2.18, 0.4 mm of pitch against 0.45. Close is the problem. An M2 bolt will go into a 2-56 hole and feel loose, because it is 0.18 mm thinner than the thread it is trying to catch, and it will strip rather than tighten. The pitches are 0.05 mm apart as well, so the threads walk out of step within a few turns. 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: #50 is 1.78 mm, and a metric drill index will offer 1.8 mm as the nearest size.
What 2-56 is used for
Model work, small enclosures and control panels.
The same 0.086" screw is also made as 2-64, the fine version, which takes a #50 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 2-56 tap?
A #50 drill, which is 0.07 inches or 1.78 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 2-56.
What size hole do I need for a 2-56 thread?
The same #50 hole — 0.07 inches. The number comes from subtracting the thread height from the major diameter: 0.086 − 0.974 ÷ 56 = 0.0686 inches, and #50 is the nearest drill that exists.
What is the 2-56 tap drill size in mm?
1.78 mm. There is no metric drill of exactly that size, so a metric drill index gives you 1.8 mm as the nearest — close enough for 2-56 in most materials, though it changes the thread engagement slightly.
Can I use a #50 instead?
Yes, and in hard material you should. #50 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
- 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
- 6-40 tap drill size — #33
- 8-32 tap drill size — #29