3BA tap drill size
The tap drill for 3BA is 3.4 mm, which gives about 80% of full thread. There’s no single standard chart size, so this is worked out from the thread profile for 75%.
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Tap drill by thread percentage
75% is the usual workshop figure. Less thread is easier to tap; more adds little strength.
| Thread % | Ideal drill | Nearest metric drill | Nearest imperial drill |
|---|---|---|---|
| 60% | 3.574 mm 0.1407" | 3.6 mm (57%) | 9/64" 0.1406" (60%) |
| 65% | 3.531 mm 0.1390" | 3.5 mm (68%) | #28 0.1405" (61%) |
| 70% | 3.487 mm 0.1373" | 3.5 mm (68%) | #29 0.1360" (74%) |
| 75% | 3.443 mm 0.1356" | 3.4 mm (80%) | #29 0.1360" (74%) |
| 80% | 3.399 mm 0.1338" | 3.4 mm (80%) | #29 0.1360" (74%) |
About this size
BA threads have a 47½° angle and are worked out from a formula (pitch = 0.9ⁿ mm). They are still used in model engineering, electrical and instrument work.
- For stainless steel, duplex and other tough materials, go to about 65% thread: a 3.5 mm drill gives 68%. For mild steel, brass and cast iron the 3.4 mm is fine.
- For a blind hole with 1.5 × D thread engagement (6.1 mm), drill about 9.1 mm deep. That leaves room for the 3–4 tapered lead threads on a second or plug tap. In steel, 1 × D engagement is usually enough; in aluminium or cast iron allow 1.5–2 × D.
Clearance holes for 3BA
| Fit | Drill |
|---|---|
| Close | 4.3 mm (0.169") |
| Medium (normal) | 4.5 mm (0.177") |
| Free (loose) | 4.9 mm (0.193") |
Worked out with our own rule, not taken from a standards table: close = the larger of D + 4% and D + 0.2 mm, medium = the larger of D + 8% and D + 0.4 mm, free = the larger of D + 15% and D + 0.75 mm, each rounded up to the next 0.1 mm. Check against your drawing where the fit matters.
3BA thread data
| Designation | 3BA |
|---|---|
| Standard | BS 93 British Association (47½°) |
| Pitch | 0.73 mm pitch (34.8 TPI equivalent) |
| Thread angle | 47½° |
| Major diameter | 4.1 mm (0.161") |
| Pitch (effective) diameter | 3.662 mm (0.144") |
| Minor diameter, external | 3.224 mm (0.127") |
| Minor diameter, internal | 3.224 mm (0.127") |
| Thread depth (internal) | 0.438 mm |
Basic sizes. Real screws are slightly under and real nuts slightly over these values because of tolerance classes.
Quick calculator
Nearest alternatives
If you don’t have the right drill, these give:
- 3.3 mm drill → about 91% thread
- 3.4 mm drill → about 80% thread
- 3.5 mm drill → about 68% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- #8 UNC – 4.17 mm major, 32 TPI (0.794 mm pitch)
- #8 UNF – 4.17 mm major, 36 TPI (0.706 mm pitch)
- M4 – 4 mm major, 0.7 mm pitch (36.3 TPI equivalent)
Questions
What size drill for a 3BA tap?
The tap drill for 3BA is 3.4 mm, which gives about 80% of full thread. There’s no single standard chart size, so this is worked out from the thread profile for 75%.
What size clearance hole for 3BA?
For a 3BA screw or bolt, drill 4.3 mm for a close fit, 4.5 mm for a medium (normal) fit or 4.9 mm for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 3BA?
3BA has a 0.73 mm pitch (34.8 TPI equivalent), a 47½° thread angle and a 4.1 mm major diameter.
Sources: Worked out from the BS 93 formulas (pitch = 0.9ⁿ mm, D = 6p^1.2, depth 0.6p). Values are for reference only – check your tap maker’s chart. See the disclaimer.
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