Squirrelly 8400 18mm Mild Steel Allen Bolt Cap for O2 Sensor Exhaust Bungs Plug The bung is welded into the exhuast and the O2 sensor (Narrowband or Wideband) screws into the bung allowing the sensor tip to be exposed to the exhaust gas in the pipe. Squirrelly Performance Parts Mild Steel Plug (18mm) is used for placement of a O2 sensor in the vehicles exhaust system. Allen bolt. 18mm. Made in the USA!