9BA tap drill size
The tap drill for 9BA is 1.55 mm, which gives about 75% 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% | 1.619 mm 0.0637" | 1.6 mm (64%) | #52 0.0635" (61%) |
| 65% | 1.596 mm 0.0628" | 1.6 mm (64%) | 1/16" 0.0625" (67%) |
| 70% | 1.572 mm 0.0619" | 1.55 mm (75%) | 1/16" 0.0625" (67%) |
| 75% | 1.549 mm 0.0610" | 1.55 mm (75%) | #53 0.0595" (83%) |
| 80% | 1.526 mm 0.0601" | 1.55 mm (75%) | #53 0.0595" (83%) |
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.
- Taps this small snap easily. By hand, or in stainless steel, a 1.6 mm drill (about 64% thread) makes tapping much easier for very little loss of strength.
- For a blind hole with 1.5 × D thread engagement (2.8 mm), drill about 4.4 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 9BA
| Fit | Drill |
|---|---|
| Close | 2.1 mm (0.083") |
| Medium (normal) | 2.3 mm (0.091") |
| Free (loose) | 2.7 mm (0.106") |
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.
9BA thread data
| Designation | 9BA |
|---|---|
| Standard | BS 93 British Association (47½°) |
| Pitch | 0.39 mm pitch (65.1 TPI equivalent) |
| Thread angle | 47½° |
| Major diameter | 1.9 mm (0.075") |
| Pitch (effective) diameter | 1.666 mm (0.066") |
| Minor diameter, external | 1.432 mm (0.056") |
| Minor diameter, internal | 1.432 mm (0.056") |
| Thread depth (internal) | 0.234 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:
- 1.5 mm drill → about 85% thread
- 1.55 mm drill → about 75% thread
- 1.6 mm drill → about 64% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- M2 – 2 mm major, 0.4 mm pitch (63.5 TPI equivalent)
Questions
What size drill for a 9BA tap?
The tap drill for 9BA is 1.55 mm, which gives about 75% 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 9BA?
For a 9BA screw or bolt, drill 2.1 mm for a close fit, 2.3 mm for a medium (normal) fit or 2.7 mm for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 9BA?
9BA has a 0.39 mm pitch (65.1 TPI equivalent), a 47½° thread angle and a 1.9 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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