This adapter kit is used to fill sodastream cylinders with Tr21-4 thread from a large CO2 tank with W21.8-14 thread. With pressure gauge 2000 psi, relief valve, quick coupler 8 mm Input: W21.8-14 thread (gold connector) Filling: Tr21-4 thread (black adapter) Material: brass and aluminum How does it work? Put a small bottle in the fridge to cool so that it can be filled with more carbon dioxide. Make sure that the on/off valve (90 degrees relative to the ball valve body) and the relief valve are closed, then turn the pin knob (orange cap) upwards (counterclockwise). Insert the white gasket into the gold connector, then connect it to the large CO2 tank (tighten with a wrench). Connect a small sodastream cylinder to a black adapter. Turn the pin knob down to gently touch the cylinder pin (clockwise). (If you push the stem too hard, it will stop the gas supply through the check valve when filling the 6 mm wide cylinder.) Open the large CO2 tank valve. Slowly open the on/off valve (counterclockwise) and close it after 2-3 seconds, then open the vent valve to release the remaining gaz in the small cylinder, then remember to close the vent valve after this step). Carefully open the on/off valve (20-30 degrees counterclockwise) to carry out the charging process for 30-60 seconds, then close the large carbon dioxide tank valve (make sure the on/off valve is open) After filling, position the pin knob in the highest position and open the vent valve to release the remaining gaz in this element. After inflation, unscrew the small cylinder. Repeat the filling process for each cylinder.-Ready. *package contains only 1 pieces. adapter set, excluding tanks