G1 (1" BSP) tap drill size
The tap drill for G1 BSPP parallel (11 TPI) is 30.75 mm. That’s the standard chart size and gives about 85% 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% | 31.475 mm 1.2392" | 31.5 mm (59%) | 1-15/64" 1.2344" (64%) |
| 65% | 31.327 mm 1.2333" | 31.25 mm (68%) | 1-15/64" 1.2344" (64%) |
| 70% | 31.179 mm 1.2275" | 31.25 mm (68%) | 1-15/64" 1.2344" (64%) |
| 75% | 31.031 mm 1.2217" | 31 mm (76%) | 1-7/32" 1.2188" (78%) |
| 80% | 30.883 mm 1.2159" | 31 mm (76%) | 1-7/32" 1.2188" (78%) |
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 31.25 mm drill gives 68%. For mild steel, brass and cast iron the 30.75 mm is fine.
- The usual 30.75 mm drill gives a deep thread (about 85%). That needs more tapping torque; if the tap is tight, open the hole one drill size.
- At this size many people bore the hole and screwcut the thread on the lathe instead of using a hand tap. See the thread calculator for depths and the infeed.
- For a blind hole with 1.5 × D thread engagement (49.9 mm), drill about 59.1 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 fitting through a panel, measure the thread’s outside diameter (33.25 mm) and drill or bore about 0.5–1 mm over it.
G1 (1" BSP) thread data
| Designation | G1 BSPP parallel (11 TPI) |
|---|---|
| Standard | ISO 228-1 (BSPP / G) |
| Pitch | 11 TPI (2.309 mm pitch) |
| Thread angle | 55° |
| Major diameter | 33.249 mm (1.309") |
| Pitch (effective) diameter | 31.77 mm (1.251") |
| Minor diameter, external | 30.292 mm (1.193") |
| Minor diameter, internal | 30.292 mm (1.193") |
| Thread depth (internal) | 1.479 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:
- 30.75 mm drill → about 85% thread
- 31 mm drill → about 76% thread
- 31.25 mm drill → about 68% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- M33 – 33 mm major, 3.5 mm pitch (7.3 TPI equivalent)
- M36 – 36 mm major, 4 mm pitch (6.3 TPI equivalent)
- M36 × 3 – 36 mm major, 3 mm pitch (8.5 TPI equivalent)
- M30 – 30 mm major, 3.5 mm pitch (7.3 TPI equivalent)
- M30 × 2 – 30 mm major, 2 mm pitch (12.7 TPI equivalent)
Questions
What size drill for a G1 (1" BSP) tap?
The tap drill for G1 BSPP parallel (11 TPI) is 30.75 mm. That’s the standard chart size and gives about 85% of full thread.
What is the pitch of G1 (1" BSP)?
G1 BSPP parallel (11 TPI) has a 11 TPI (2.309 mm pitch), a 55° thread angle and a 33.249 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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