AGPTEK Oxygen Acetylene Flashback Arrestors Set Regulator End 9/16 -18 Threads B Size Flashback arrestor set has internal check valves for both oxygen and acetylene Standard -InchB-Inch fittings for 9/16 -18 threads,B size. Attach at the regulator. NOTE: The acetylene arrestor in this set can also be used for Propane or Mapp gas.