Tap drill size for 4-48
#42
0.0935 inches · 2.37 mm · 75% thread engagement
| Tap drill (75% thread) | #42 — 0.0935" / 2.37 mm |
| Major diameter | 0.112" / 2.84 mm |
| Drill for a 100% thread | 0.0849" / 2.16 mm |
| Threads per inch | 48 |
| Pitch | 0.0208" / 0.53 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:
0.112 − 1.299 ÷ 48 = 0.0849 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:
0.112 − 0.974 ÷ 48 = 0.0917 inches
The nearest drill that actually exists is #42 at 0.0935 inches, which is 0.0018 inches over the theoretical figure. That is the number printed on every 4-48 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% | 0.0985" | #40 (0.098") | 2.49 mm |
| 65% | 0.0944" | 3/32" (0.0938") | 2.38 mm |
| 75% — the chart value | 0.0917" | #42 (0.0935") | 2.37 mm |
| 85% | 0.089" | #43 (0.089") | 2.26 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.
4-48 in metric
4-48 has a major diameter of 2.84 mm and a pitch of 0.53 mm. The closest metric tap is M3, and it is close: 3 mm across against 2.84, 0.5 mm of pitch against 0.53. Close is the problem. An M3 bolt will start in a 4-48 hole and then jam, because it is 0.16 mm fatter than the hole was cut for. The pitches are near enough identical — 0.5 mm against 0.53 — 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: #42 is 2.37 mm, and a metric drill index will offer 2.4 mm as the nearest size.
What 4-48 is used for
Fine-pitch #4, common in aerospace and instrument work.
The same 0.112" screw is also made as 4-40, the coarse version, which takes a #43 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 4-48 tap?
A #42 drill, which is 0.0935 inches or 2.37 mm. That is the standard 75% thread engagement figure, and it is what every published tap chart gives for 4-48.
What size hole do I need for a 4-48 thread?
The same #42 hole — 0.0935 inches. The number comes from subtracting the thread height from the major diameter: 0.112 − 0.974 ÷ 48 = 0.0917 inches, and #42 is the nearest drill that exists.
What is the 4-48 tap drill size in mm?
2.37 mm. There is no metric drill of exactly that size, so a metric drill index gives you 2.4 mm as the nearest — close enough for 4-48 in most materials, though it changes the thread engagement slightly.
Can I use a 3/32" instead?
Yes, and in hard material you should. 3/32" 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
- 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
- 8-36 tap drill size — #29
- 10-24 tap drill size — #25
- 10-32 tap drill size — #21
- 12-24 tap drill size — #16
- 12-28 tap drill size — #14