What is the difference between BSP parallel and BSP taper threads?

BSP Parallel (BSPP, also marked G) threads have a constant diameter along their length and require a washer, O-ring or bonded seal to make a watertight joint. BSP Taper (BSPT, also marked R or Rp) threads taper at 1 in 16 (1°47'24") and create a seal by metal-to-metal contact as the taper compresses on tightening, usually with PTFE tape or thread sealant. In UK plumbing, taper male into parallel female (Rp) is the most common combination.

What does the BSP size number refer to?

BSP thread sizes (1/4", 1/2", 3/4", 1" etc.) refer to the approximate bore of the pipe, not the outside diameter of the thread. For example, a 1/2" BSP thread has a major outside diameter of approximately 20.95mm, not 12.7mm. This is a historical convention from when pipe wall thicknesses were greater. The nominal bore is always significantly smaller than the thread OD.

Are BSP and NPT threads compatible?

No, BSP and NPT (National Pipe Thread, used in the USA) are not compatible. They have different thread angles (55° for BSP vs 60° for NPT) and different taper rates. While some sizes may appear to start threading together, they will not seal properly and will leak or cross-thread. Always verify whether fittings are BSP or NPT before connecting — adapters are available if needed.

What thread sealant should I use on BSP threads?

For taper threads (BSPT/R): PTFE tape (wrapped clockwise when looking at the thread end, 5-8 wraps) or liquid thread sealant (e.g., Loctite 55, Boss White). For parallel threads (BSPP/G): a fibre washer, bonded seal (Dowty washer) or O-ring provides the seal — PTFE tape alone is not sufficient as parallel threads cannot create a taper compression seal.

BSP Pipe Thread Sizes — British Standard Pipe

Complete BSP (British Standard Pipe) thread reference including parallel (BSPP/G) and taper (BSPT/R) thread dimensions, TPI, major diameters and tapping drill sizes.

BSP Thread Dimensions — Parallel (BSPP / G)

Reference: BS EN ISO 228-1. Thread angle 55° (Whitworth form).

Nominal Size	TPI	Major OD (mm)	Minor ID (mm)	Pitch (mm)	Tapping Drill (mm)
1/8"	28	9.728	8.566	0.907	8.6
1/4"	19	13.157	11.445	1.337	11.5
3/8"	19	16.662	14.950	1.337	15.0
1/2"	14	20.955	18.631	1.814	18.6
3/4"	14	26.441	24.117	1.814	24.1
1"	11	33.249	30.291	2.309	30.3
1 1/4"	11	41.910	38.952	2.309	39.0
1 1/2"	11	47.803	44.845	2.309	44.8
2"	11	59.614	56.656	2.309	56.7

BSP Taper Thread (BSPT / R) — Key Differences

Feature	BSP Parallel (BSPP/G)	BSP Taper (BSPT/R)
Thread profile	Constant diameter	Tapers at 1:16
Thread angle	55°	55°
Sealing method	Washer, O-ring or bonded seal	Metal-to-metal + PTFE tape/sealant
Standard	BS EN ISO 228-1	BS 21 / BS EN 10226
Designation	G (e.g., G 1/2)	R (male) / Rp (female parallel for taper male)
Common use	Hydraulics, instrumentation	Plumbing, gas, compressed air

Common Plumbing Thread Combinations (UK)

1/2" BSP — Most common UK tap, valve, and fitting size. Basin taps, bath taps, radiator valves, flexible connectors.
3/4" BSP — Washing machine valves, outside taps, bath mixer feeds, larger radiator connections.
1" BSP — Main stopcock, gas meter connections, larger heating circuits.
1 1/4" and 1 1/2" BSP — Waste fittings (basin and bath traps), larger heating mains.
2" BSP — Larger commercial pipe connections, pumps.

See the UK Pipe Sizes reference and the UK Copper Pipe Sizes guide.