When the mating threads of a cylinder valve and a regulator do not allow for a direct connection, the use of an adaptor is needed. There are 4 different acetylene CGA cylinder valves and CGA acetylene regulators in the United States; CGA-200, CGA-300, CGA-510, and CGA-520.CGA-200 is found in Size MC Tanks. CGA-520 is found in Size B Tanks. CGA-510 is found in almost all larger Acetylene and Propane tanks. CGA-300 is also used in larger Acetylene tanks but is not common. Please carefully check your acetylene cylinder and make sure you know the type of valve installed.Carefully check the nipple and nut installed in your regulator and make sure you know what CGA standard it has. It Is usually marked in the nut. If not, check all the pictures of this listing and compare the dimensions.All SÜA Cylinder-to-Regulator Gas adaptors are made from durable brass. Threads are carefully measured to ensure a snug and tight seal in all connections. Adaptors with welded parts may show welding material covering some areas outside the joint, this will not affect the performance of these adaptors.Select the right adaptor from the "Size" menu options. The first number is the cylinder´s valve size and the second number is the regulator´s size. FOR EXAMPLE: "CGA-200 to CGA-520" is an adaptor that will connect a CGA-200 Cylinder Valve to a CGA-510 Regulator. After selecting your option, check the dimensions on Picture #2 and make sure it will fit your regulator and tanks´s valve.