G1/8 (1/8" BSP) tap drill size
The tap drill for G1/8 BSPP parallel (28 TPI) is 8.8 mm. That’s the standard chart size and gives about 80% 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% | 9.031 mm 0.3555" | 9 mm (63%) | letter T 0.3580" (55%) |
| 65% | 8.973 mm 0.3533" | 9 mm (63%) | letter T 0.3580" (55%) |
| 70% | 8.915 mm 0.3510" | 8.9 mm (71%) | letter S 0.3480" (77%) |
| 75% | 8.857 mm 0.3487" | 8.9 mm (71%) | letter S 0.3480" (77%) |
| 80% | 8.799 mm 0.3464" | 8.8 mm (80%) | letter S 0.3480" (77%) |
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 9 mm drill gives 63%. For mild steel, brass and cast iron the 8.8 mm is fine.
- For a blind hole with 1.5 × D thread engagement (14.6 mm), drill about 18.2 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 G1/8 fitting through a panel, measure the thread’s outside diameter (9.73 mm) and drill or bore about 0.5–1 mm over it.
G1/8 (1/8" BSP) thread data
| Designation | G1/8 BSPP parallel (28 TPI) |
|---|---|
| Standard | ISO 228-1 (BSPP / G) |
| Pitch | 28 TPI (0.907 mm pitch) |
| Thread angle | 55° |
| Major diameter | 9.728 mm (0.383") |
| Pitch (effective) diameter | 9.147 mm (0.360") |
| Minor diameter, external | 8.566 mm (0.337") |
| Minor diameter, internal | 8.566 mm (0.337") |
| Thread depth (internal) | 0.581 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:
- 8.8 mm drill → about 80% thread
- 8.9 mm drill → about 71% thread
- 9 mm drill → about 63% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- 3/8" UNC – 9.52 mm major, 16 TPI (1.587 mm pitch)
- 3/8" UNF – 9.52 mm major, 24 TPI (1.058 mm pitch)
- 3/8" BSW – 9.52 mm major, 16 TPI (1.587 mm pitch)
- 3/8" BSF – 9.52 mm major, 20 TPI (1.27 mm pitch)
- M10 – 10 mm major, 1.5 mm pitch (16.9 TPI equivalent)
Questions
What size drill for a G1/8 (1/8" BSP) tap?
The tap drill for G1/8 BSPP parallel (28 TPI) is 8.8 mm. That’s the standard chart size and gives about 80% of full thread.
What is the pitch of G1/8 (1/8" BSP)?
G1/8 BSPP parallel (28 TPI) has a 28 TPI (0.907 mm pitch), a 55° thread angle and a 9.728 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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