#6-40 Tap Drill Size
The tap drill for #6-40 UNF is #33 (0.1130 in, 2.87 mm). That is the standard chart size and leaves about 77% thread.
Drill a #33 hole, then cut the thread with a #6-40 tap. The hole is 0.0250 in narrower than the screw, so the tap cuts 0.0125 in into each side.
A full-height thread would be 0.0162 in deep on each side. The #33 drill leaves 77% of that.
Try another material or drill
#6-40 tap drill size by material
| Material | Target thread | Drill | Size | Thread left |
|---|---|---|---|---|
| Aluminum, brass, plastics | 75% | #33 | 0.1130 in (2.87 mm) | 77% |
| Mild steel | 60% | #31 | 0.1200 in (3.05 mm) | 55% |
| Stainless, titanium | 50% | #31 | 0.1200 in (3.05 mm) | 55% |
The 75% row is the published chart drill. The 60% and 50% rows pick the biggest standard drill that still leaves close to that much thread.
Drill sizes are coarse at this thread, so steel and stainless land on the same drill. How these are worked out.
How much thread do you need?
The chart prints one drill. The right amount of thread depends on the material, and the guides give slightly different ranges.
| Material | Texas Metalworks | ToolGrit | OSG (tap maker) | Drill for #6-40 |
|---|---|---|---|---|
| Aluminum | 70-75% | 60-65% | 65-70% | #33 (77%) to #31 (55%) |
| Brass | 70-75% | 70-75% | 65-70% | #33 (77%) |
| Plastic | not covered | 70-75% | not covered | #33 (77%) |
| Mild steel | 60-65% | 60-65% | 65-70% | #31 (55%) |
| Stainless steel | 50% | 55-60% | 60-70% | #31 (55%) to #31 (55%) |
| Titanium | 50% | 50-55% | not covered | #31 (55%) |
The three rows in the first table follow Texas Metalworks. OSG makes taps, and asks for more thread in stainless than either guide, so its range sits closer to our steel column. ToolGrit also suggests 50 to 55% for blind holes in stainless, titanium and hardened steel, and for small taps (#6 and below), which includes #6-40.
Why not a full thread?
A 100% thread is not possible in practice. The tap and the thread both have flat crests and roots (ToolGrit).
More thread also means more work for the tap: higher cutting force, more heat, poorer chip clearance and a bigger risk of breaking it (ToolGrit).
And it adds little strength. ToolGrit cites studies showing about 90% of a thread's shear strength at 60% thread. Texas Metalworks says going from 75% to 60% costs only about 5% of pull-out strength.
On #6-40, that step is from #33 to #31. The two drills differ by 0.0070 in.
If you do not have the #33 drill
Here are the drills on either side, and the thread each one leaves.
| Drill | Size | Thread left | What changes |
|---|---|---|---|
| #35 | 0.1100 in (2.79 mm) | 86% | More thread, harder to tap |
| #34 | 0.1110 in (2.82 mm) | 83% | More thread, harder to tap |
| #33 | 0.1130 in (2.87 mm) | 77% | Standard size |
| #32 | 0.1160 in (2.95 mm) | 68% | Less thread, easier to tap |
| #31 | 0.1200 in (3.05 mm) | 55% | Less thread, easier to tap |
A bigger drill removes thread and makes the tap easier to turn. A smaller drill keeps more thread and puts more torque on the tap.
The check behind the #33 number: (0.1380 - 0.1130) / (1.299 x 0.0250) = 77.0%.
What #6-40 means
- #6
- A screw gauge number. The major diameter is 0.060 + 0.013 x 6 = 0.1380 in (3.51 mm).
- 40
- Threads per inch, so one thread every 0.0250 in (0.635 mm).
- UNF
- Unified National Fine. Its coarse twin is #6-32.
The rest of the size, worked out from the major diameter and pitch:
| Property | Value |
|---|---|
| Major diameter | 0.1380 in (3.51 mm) |
| Pitch | 40 TPI (0.635 mm) |
| Pitch diameter (basic) | 0.1218 in (3.09 mm) |
| Minor diameter, internal thread (basic) | 0.1109 in (2.82 mm) |
The pitch and minor diameters are for checking a thread with gauges. You can skip them if you only need the drill.
How long to make the #6-40 thread
Aim for a thread at least 1 to 1.5 times the diameter long. ToolGrit's strength figures assume that length.
| Thread length | In threads | Drill a blind hole at least |
|---|---|---|
| 1 x diameter: 0.138 in | 5.5 | 0.238 in |
| 1.5 x diameter: 0.207 in | 8.3 | 0.307 in |
The blind hole depth adds 4 threads (4 x 0.0250 in) so the tap has room to finish and the chips have somewhere to go.
How to tap a #6-40 hole
- Drill the hole with the #33 drill (0.1130 in, 2.87 mm).
- For a blind hole, drill 0.238 in deep if you need a 0.138 in thread. That is 4 threads past the thread (ToolGrit).
- Put cutting fluid on the tap. Do not tap dry (ToolGrit, Wikipedia).
- Hold the tap in line with the hole.
- Turn the tap forward, then back it off now and then to break the chip (Wikipedia).
- Need threads right to the bottom of a blind hole? Finish with a bottoming tap (Wikipedia).
Taps usually break for one of these reasons (Wikipedia, ToolGrit):
- Chips clog the flutes.
- The tap is not in line with the hole.
- The tap is pushed too hard or too little.
- There is not enough cutting fluid.
- The drill was too small, so the tap works against too much thread.
Which tap to use for #6-40
The three common hand taps differ in how many threads at the tip are tapered (Wikipedia).
- Taper tap: 8 to 10 tapered threads, about 0.2000 in to 0.2500 in on #6-40. For hard materials and very small taps.
- Plug tap: 3 to 5 tapered threads, about 0.0750 in to 0.1250 in on #6-40. The most common tap, easy to start and keep straight.
- Bottoming tap: 1 to 1.5 tapered threads, about 0.0250 in to 0.0375 in on #6-40. Cuts threads to the bottom of a blind hole, usually after a plug tap.
Spiral-point taps are usually used in through holes so the chips can escape. Spiral-flute taps work well in blind holes (Wikipedia).
Forming (roll) taps only work in malleable metals such as mild steel and aluminum (Wikipedia). They use different drill sizes, which we do not cover yet.
#6-40 vs #6-32
These two share the same diameter. They differ in pitch, so they take different drills.
| #6-40 | #6-32 | |
|---|---|---|
| Series | UNF (fine) | UNC (coarse) |
| Pitch | 40 TPI (0.635 mm) | 32 TPI (0.794 mm) |
| Standard drill | #33 | #36 |
| Thread depth, each side | 0.0162 in (0.41 mm) | 0.0203 in (0.52 mm) |
Thread depth is 0.6495 x pitch. Do not mix the two: a #6-40 screw will not fit a #6-32 hole.
Closest metric size
M3.5 is close to #6-40, but it is not the same thread.
| Major diameter | Pitch | Standard drill | |
|---|---|---|---|
| #6-40 | 3.51 mm | 0.635 mm | #33 |
| M3.5 | 3.50 mm | 0.600 mm | 2.9 mm |
The diameters differ by 0.01 mm and the pitches are not the same. Do not swap them.
FAQ
What is the tap drill size for #6-40?
The standard drill is #33 (0.1130 in, 2.87 mm). It leaves about 77% thread, which is the standard chart choice.
Can I use #32 instead of #33 for #6-40?
#32 (0.1160 in, 2.95 mm) leaves about 68% thread instead of 77%. The tap cuts more easily and the thread is weaker. That is still at or above the 60% Texas Metalworks suggests for mild steel.
Is a 2.9 mm drill the same as #33?
It is very close. 2.9 mm is 0.1142 in, against 0.1130 in, 2.87 mm for #33. It leaves about 73% thread.
What drill size do I use for #6-40 in aluminum?
Use #33 (77%) or #31 (55%), depending on the guide. Texas Metalworks says 70 to 75% for soft aluminum, which is #33. ToolGrit says 60 to 65% for aluminum alloys, which is #31. Either is a normal choice. OSG, a tap maker, gives 65 to 70% for average work in aluminum, brass and mild steel.
What is the tap drill size for #6-40 in stainless steel?
#31 (0.1200 in, 3.05 mm) leaves about 55% thread, close to the 50% Texas Metalworks recommends for stainless and titanium. ToolGrit gives 55 to 60% for 304 and 316 stainless, which is between #31 and #31. The standard drill, #33, puts more torque on the tap. OSG, a tap maker, asks for 60 to 70% in stainless for average work, which is closer to the steel drill, #31.
Does a bigger drill make the #6-40 thread much weaker?
Not by much at first. ToolGrit cites studies showing about 90% of a thread's shear strength at 60% thread, if the thread is at least 1 to 1.5 times the diameter long. Texas Metalworks says going from 75% to 60% costs only about 5% of pull-out strength. For #6-40, the 60% drill is #31.
How long should a #6-40 thread be?
Aim for at least 1 to 1.5 times the diameter, the length ToolGrit's strength figures assume. For #6-40 that is 0.138 in to 0.207 in, or about 5.5 to 8.3 threads.
How deep do I drill a blind #6-40 hole?
At least 4 threads deeper than the thread you need, says ToolGrit. A 0.138 in thread needs a hole at least 0.238 in deep.
Which tap do I use for a blind #6-40 hole?
Start with a plug tap, then finish with a bottoming tap if the thread has to reach the bottom. Wikipedia says a bottoming tap has 1 to 1.5 tapered threads, about 0.0250 in to 0.0375 in on #6-40.
What is #6-40 in mm?
#6-40 is 3.51 mm across, with a pitch of 0.635 mm.
Is #6-40 a coarse or fine thread?
#6-40 is a fine thread (UNF). The coarse thread of the same diameter is #6-32, which takes #36.
Related sizes
Chart drill checked against the American Fastener tap and drill chart and the Fabworks tap drill chart. Diameters from Wikipedia: drill bit sizes. Thread percentages, thread length and blind-hole depth from Texas Metalworks and ToolGrit. The percentage formula is the one printed in the OSG tap drill chart, Vol 3 (2024). Tap types and tapping tips from Wikipedia. Last reviewed October 7, 2026.