G1/4 (1/4" BSP) tap drill size
The tap drill for G1/4 BSPP parallel (19 TPI) is 11.8 mm. That’s the standard chart size and gives about 79% 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% | 12.13 mm 0.4776" | 12.1 mm (62%) | 31/64" 0.4844" (50%) |
| 65% | 12.044 mm 0.4742" | 12 mm (68%) | 15/32" 0.4688" (73%) |
| 70% | 11.959 mm 0.4708" | 12 mm (68%) | 15/32" 0.4688" (73%) |
| 75% | 11.873 mm 0.4674" | 11.9 mm (73%) | 15/32" 0.4688" (73%) |
| 80% | 11.787 mm 0.4641" | 11.8 mm (79%) | 15/32" 0.4688" (73%) |
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 12 mm drill gives 68%. For mild steel, brass and cast iron the 11.8 mm is fine.
- For a blind hole with 1.5 × D thread engagement (19.7 mm), drill about 25.1 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 hole
Pipe threads don’t take a clearance hole in the bolt sense. To pass a G1/4 fitting through a panel, measure the thread’s outside diameter (13.16 mm) and drill or bore about 0.5–1 mm over it.
G1/4 (1/4" BSP) thread data
| Designation | G1/4 BSPP parallel (19 TPI) |
|---|---|
| Standard | ISO 228-1 (BSPP / G) |
| Pitch | 19 TPI (1.337 mm pitch) |
| Thread angle | 55° |
| Major diameter | 13.157 mm (0.518") |
| Pitch (effective) diameter | 12.301 mm (0.484") |
| Minor diameter, external | 11.445 mm (0.451") |
| Minor diameter, internal | 11.445 mm (0.451") |
| Thread depth (internal) | 0.856 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:
- 11.8 mm drill → about 79% thread
- 11.9 mm drill → about 73% thread
- 12 mm drill → about 68% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- 1/2" UNC – 12.7 mm major, 13 TPI (1.954 mm pitch)
- 1/2" UNF – 12.7 mm major, 20 TPI (1.27 mm pitch)
- 1/2" BSW – 12.7 mm major, 12 TPI (2.117 mm pitch)
- 1/2" BSF – 12.7 mm major, 16 TPI (1.587 mm pitch)
- M14 – 14 mm major, 2 mm pitch (12.7 TPI equivalent)
Questions
What size drill for a G1/4 (1/4" BSP) tap?
The tap drill for G1/4 BSPP parallel (19 TPI) is 11.8 mm. That’s the standard chart size and gives about 79% of full thread.
What is the pitch of G1/4 (1/4" BSP)?
G1/4 BSPP parallel (19 TPI) has a 19 TPI (1.337 mm pitch), a 55° thread angle and a 13.157 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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