G3/8 (3/8" BSP) tap drill size
The tap drill for G3/8 BSPP parallel (19 TPI) is 15.25 mm. That’s the standard chart size and gives about 82% 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% | 15.635 mm 0.6155" | 15.75 mm (53%) | 39/64" 0.6094" (69%) |
| 65% | 15.549 mm 0.6122" | 15.5 mm (68%) | 39/64" 0.6094" (69%) |
| 70% | 15.464 mm 0.6088" | 15.5 mm (68%) | 39/64" 0.6094" (69%) |
| 75% | 15.378 mm 0.6054" | 15.5 mm (68%) | 39/64" 0.6094" (69%) |
| 80% | 15.292 mm 0.6021" | 15.25 mm (82%) | 39/64" 0.6094" (69%) |
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 15.5 mm drill gives 68%. For mild steel, brass and cast iron the 15.25 mm is fine.
- The usual 15.25 mm drill gives a deep thread (about 82%). 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 (25 mm), drill about 30.3 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 G3/8 fitting through a panel, measure the thread’s outside diameter (16.66 mm) and drill or bore about 0.5–1 mm over it.
G3/8 (3/8" BSP) thread data
| Designation | G3/8 BSPP parallel (19 TPI) |
|---|---|
| Standard | ISO 228-1 (BSPP / G) |
| Pitch | 19 TPI (1.337 mm pitch) |
| Thread angle | 55° |
| Major diameter | 16.662 mm (0.656") |
| Pitch (effective) diameter | 15.806 mm (0.622") |
| Minor diameter, external | 14.95 mm (0.589") |
| Minor diameter, internal | 14.95 mm (0.589") |
| Thread depth (internal) | 0.856 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:
- 15.25 mm drill → about 82% thread
- 15.5 mm drill → about 68% thread
- 15.75 mm drill → about 53% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- M16 – 16 mm major, 2 mm pitch (12.7 TPI equivalent)
- M16 × 1.5 – 16 mm major, 1.5 mm pitch (16.9 TPI equivalent)
- 5/8" UNC – 15.88 mm major, 11 TPI (2.309 mm pitch)
- 5/8" UNF – 15.88 mm major, 18 TPI (1.411 mm pitch)
- 5/8" BSW – 15.88 mm major, 11 TPI (2.309 mm pitch)
Questions
What size drill for a G3/8 (3/8" BSP) tap?
The tap drill for G3/8 BSPP parallel (19 TPI) is 15.25 mm. That’s the standard chart size and gives about 82% of full thread.
What is the pitch of G3/8 (3/8" BSP)?
G3/8 BSPP parallel (19 TPI) has a 19 TPI (1.337 mm pitch), a 55° thread angle and a 16.662 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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