7BA tap drill size
The tap drill for 7BA is 2.05 mm, which gives about 78% of full thread. There’s no single standard chart size, so this is worked out from the thread profile for 75%.
Open in the full tap drill finder
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% | 2.154 mm 0.0848" | 2.15 mm (61%) | #44 0.0860" (55%) |
| 65% | 2.126 mm 0.0837" | 2.15 mm (61%) | #45 0.0820" (72%) |
| 70% | 2.097 mm 0.0826" | 2.1 mm (69%) | #45 0.0820" (72%) |
| 75% | 2.068 mm 0.0814" | 2.05 mm (78%) | #46 0.0810" (77%) |
| 80% | 2.039 mm 0.0803" | 2.05 mm (78%) | #46 0.0810" (77%) |
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 2.15 mm drill (about 61% thread) makes tapping much easier for very little loss of strength.
- For a blind hole with 1.5 × D thread engagement (3.8 mm), drill about 5.7 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 7BA
| Fit | Drill |
|---|---|
| Close | 2.7 mm (0.106") |
| Medium (normal) | 2.9 mm (0.114") |
| Free (loose) | 3.3 mm (0.130") |
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.
7BA thread data
| Designation | 7BA |
|---|---|
| Standard | BS 93 British Association (47½°) |
| Pitch | 0.48 mm pitch (52.9 TPI equivalent) |
| Thread angle | 47½° |
| Major diameter | 2.5 mm (0.098") |
| Pitch (effective) diameter | 2.212 mm (0.087") |
| Minor diameter, external | 1.924 mm (0.076") |
| Minor diameter, internal | 1.924 mm (0.076") |
| Thread depth (internal) | 0.288 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:
- 2 mm drill → about 87% thread
- 2.05 mm drill → about 78% thread
- 2.1 mm drill → about 69% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- M2.5 – 2.5 mm major, 0.45 mm pitch (56.4 TPI equivalent)
Questions
What size drill for a 7BA tap?
The tap drill for 7BA is 2.05 mm, which gives about 78% 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 7BA?
For a 7BA screw or bolt, drill 2.7 mm for a close fit, 2.9 mm for a medium (normal) fit or 3.3 mm for a free fit. These come from our stated rule, not a standards table.
What is the pitch of 7BA?
7BA has a 0.48 mm pitch (52.9 TPI equivalent), a 47½° thread angle and a 2.5 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.
Spotted a mistake? Report an error.