G1/2 (1/2" BSP) tap drill size
The tap drill for G1/2 BSPP parallel (14 TPI) is 19 mm. That’s the standard chart size and gives about 84% of full thread.
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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% | 19.561 mm 0.7701" | 19.5 mm (63%) | 49/64" 0.7656" (65%) |
| 65% | 19.445 mm 0.7655" | 19.5 mm (63%) | 49/64" 0.7656" (65%) |
| 70% | 19.329 mm 0.7610" | 19.25 mm (73%) | 49/64" 0.7656" (65%) |
| 75% | 19.212 mm 0.7564" | 19.25 mm (73%) | 3/4" 0.7500" (82%) |
| 80% | 19.096 mm 0.7518" | 19 mm (84%) | 3/4" 0.7500" (82%) |
About this size
G (BSPP) is a parallel pipe thread with the 55° Whitworth form. It does not seal on the thread itself: the joint is made by a washer, O-ring or bonded seal. The nominal size is the old pipe bore, not the thread diameter.
- For stainless steel, duplex and other tough materials, go to about 65% thread: a 19.5 mm drill gives 63%. For mild steel, brass and cast iron the 19 mm is fine.
- The usual 19 mm drill gives a deep thread (about 84%). That needs more tapping torque; if the tap is tight, open the hole one drill size.
- For a blind hole with 1.5 × D thread engagement (31.4 mm), drill about 38.7 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 hole
Pipe threads don’t take a clearance hole in the bolt sense. To pass a G1/2 fitting through a panel, measure the thread’s outside diameter (20.95 mm) and drill or bore about 0.5–1 mm over it.
G1/2 (1/2" BSP) thread data
| Designation | G1/2 BSPP parallel (14 TPI) |
|---|---|
| Standard | ISO 228-1 (BSPP / G) |
| Pitch | 14 TPI (1.814 mm pitch) |
| Thread angle | 55° |
| Major diameter | 20.955 mm (0.825") |
| Pitch (effective) diameter | 19.793 mm (0.779") |
| Minor diameter, external | 18.632 mm (0.734") |
| Minor diameter, internal | 18.632 mm (0.734") |
| Thread depth (internal) | 1.162 mm |
Basic sizes. Real screws are slightly under and real nuts slightly over these values because of tolerance classes.
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Nearest alternatives
If you don’t have the right drill, these give:
- 19 mm drill → about 84% thread
- 19.25 mm drill → about 73% thread
- 19.5 mm drill → about 63% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- M20 – 20 mm major, 2.5 mm pitch (10.2 TPI equivalent)
- M20 × 1.5 – 20 mm major, 1.5 mm pitch (16.9 TPI equivalent)
- M22 – 22 mm major, 2.5 mm pitch (10.2 TPI equivalent)
- M22 × 1.5 – 22 mm major, 1.5 mm pitch (16.9 TPI equivalent)
- 7/8" UNC – 22.22 mm major, 9 TPI (2.822 mm pitch)
Questions
What size drill for a G1/2 (1/2" BSP) tap?
The tap drill for G1/2 BSPP parallel (14 TPI) is 19 mm. That’s the standard chart size and gives about 84% of full thread.
What is the pitch of G1/2 (1/2" BSP)?
G1/2 BSPP parallel (14 TPI) has a 14 TPI (1.814 mm pitch), a 55° thread angle and a 20.955 mm major diameter.
Sources: Basic sizes per ISO 228-1. Chart drill from the common G (BSPP) tapping chart. Values are for reference only – check your tap maker’s chart. See the disclaimer.
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