M18 O2 Sensor Weld Bungs From:

If you are making custom headers, or just require a O2 Sensor in your system for data logging, engine management or test equipment, we have these weld on M18 x 1.5 weld fittings designed to accept the commonly found and used M18 O2 Sensors. 

These are CNC machine at Speedflow's factory in Taree, NSW in a choice of Steel, Stainless Steel or Aluminium.

We also have the plugs available separately, check out our steel 806 metric plugs here, Plugs & Caps