Squirrelly 304 Stainless Steel O2 Sensor Exhaust Bung and Mild Steel Plug Kit Squirrelly Performance Parts Stainless-Steel Bung and Plug kit (18mm x 1.25) is used for placement of a O2 sensor in the vehicles exhaust system. 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. The Bung is made of Stainless Steel while the Plug is Mild-Steel.