The cap fits most barrels with a 2" neck to enable the injection of gas using a SodaStream style CO2 Cylinder. Supplied ready to use with fitted o-ring. This valve is fitted with a pin so it is suitable for using small 8g bulbs, The valve has a 2 way function, allowing the addition of gas and the release of excess pressure. Dimensions: 8cm (body width) x 5cm (height)