BS 341 No.8 CO2 valve: 21.8 mm x 14 TPI right hand, external (the W21.8 thread), cylinder valve outlet nut, external, seals on captive O-ring or washer in the nut against the valve outlet. What it fits and the adapters to join it to other keg fittings.
Updated 05 September 2026.
BS 341 No.8 CO2 valve (W21.8 (British designation))
Where W21.8 (British designation) is used, its real dimensions, and how to connect it to other keg fittings.
Published dimensions
- Designation
- W21.8 (British designation)
- Size / thread
- 21.8 mm x 14 TPI right hand, external (the W21.8 thread)
- Fitting form
- cylinder valve outlet nut, external
- Seals on
- captive O-ring or washer in the nut against the valve outlet
- Rating / role
- the British Standard 341 designation for the CO2 cylinder outlet used across the UK
Used on: UK CO2 cylinders and older British draft kit that labels the outlet as BS 341 No.8.
BS 341 No.8 is the British name for the same W21.8 CO2 outlet used across Europe. A W21.8 or DIN 477 No.6 regulator fits it directly. It is not the same as the North American CGA-320, so a US regulator still needs a CGA-320 to W21.8 adapter to run on a UK cylinder.
Flare fittings seal on the 45 degree flare face and must never be taped. Push-in fittings seal on an O-ring against the tube wall and need the tube cut square. Barbs seal under a worm clamp. Getting the sealing method right matters more than the thread label, because most homebrew leaks are a taped flare or an unclamped barb.
Connect this fitting
Questions and answers
What is the designation of the BS 341 No.8 CO2 valve (W21.8 (British designation))?
W21.8 (British designation)
What is the size / thread of the BS 341 No.8 CO2 valve (W21.8 (British designation))?
21.8 mm x 14 TPI right hand, external (the W21.8 thread)
What is the fitting form of the BS 341 No.8 CO2 valve (W21.8 (British designation))?
cylinder valve outlet nut, external
What is the seals on of the BS 341 No.8 CO2 valve (W21.8 (British designation))?
captive O-ring or washer in the nut against the valve outlet