Threads are carefully measured to ensure a snug and tight seal in all connections. After selecting your option, check the dimensions on Picture #2 and make sure it will fit your regulator and tanks´s valve. FOR EXAMPLE: CGA-200 to CGA-520 is an adaptor that will connect a CGA-200 Cylinder Valve to a CGA-510 Regulator. CGA-300 is also used in larger Acetylene tanks but is not common. Oxygen Acetylene Torch Kit Brass Cylinder Adaptors,Cga-520 B Tank Acetylene To Cga-300 Commercial Tank 510 If not, check all the pictures of this listing and compare the dimensions. Select the right adaptor from the Size menu options. Adaptors with welded parts may show welding material covering some areas outside the joint, this will not affect the performance of these adaptors. 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 Picture #3 and make sure it will fit your regulator and tankss valve All SÜA Cylinder-to-Regulator Gas adaptors are made from durable brass. CGA-520 is found in Size B Tanks. The first number is the cylinder´s valve size and the second number is the regulator´s size. CGA-510 is found in almost all larger Acetylene and Propane tanks.