The Whitworth-form counterpart to NPT: a 55 degree thread on the same 1:16 taper, external (R). It is not interchangeable with NPT -- the flank angle and pitch diameters differ even where a nominal size lines up.
Standard
ISO 7-1 / BS 21
Flank angle
55° included
Taper
1 : 16 on diameter
Sealing
Thread-flank interference on the 1:16 taper, the same principle as NPT but with the rounded 55 degree Whitworth profile.
A 55° included flank angle with a rounded crest and root -- the single fastest way to tell a Whitworth-derived (BSP) thread from the other family by eye.
1 : 16 taper on diameter (0.75″ per foot of length)Angle exaggerated for visibility — the real taper is far shallower.
Size chart
Basic dimensions, BSPT / R · tap drill at 75% thread
Nominal
TPI
Pitch
Major Ø
Pitch Ø
Minor Ø
Tap drill
1/8
28
0.0357"
0.3830"
0.3601"
0.3373"
S (0.3487")
1/4
19
0.0526"
0.5180"
0.4843"
0.4506"
15/32" (0.4674")
3/8
19
0.0526"
0.6560"
0.6223"
0.5886"
39/64" (0.6054")
1/2
14
0.0714"
0.8250"
0.7793"
0.7335"
3/4" (0.7564")
3/4
14
0.0714"
1.0410"
0.9953"
0.9495"
31/32" (0.9724")
1
11
0.0909"
1.3090"
1.2508"
1.1926"
1-7/32" (1.2217")
1-1/4
11
0.0909"
1.6500"
1.5918"
1.5336"
1-9/16" (1.5627")
1-1/2
11
0.0909"
1.8820"
1.8238"
1.7656"
1-51/64" (1.7947")
2
11
0.0909"
2.3470"
2.2888"
2.2306"
2" (2.2597")
Tap drill calculator
BSPT / R is tapered: engagement here depends on how far the joint is driven onto the taper, not diameter alone. Treat this as a straight-thread approximation and check ISO 7-1 / BS 21 or your tap manufacturer’s chart before cutting production parts.
Calculated live
Tap drillS0.3487" calculated · S (0.3487") nearest standard
MAN4MACHINE