A stainless weld-in bung threaded M18 x 1.5, the standard oxygen sensor thread. Any custom manifold, downpipe or full exhaust needs a sensor location, and on wideband tuning work the position of that bung directly affects the quality of the reading you get. Stainless is the right material choice here, sitting in the exhaust stream where a mild steel bung eventually corrodes and seizes the sensor in place. Weld it upstream enough to read hot gas but angled upward from horizontal, so condensation cannot pool against the sensor tip and crack it on a cold start. Fit a blanking plug if the port is not in use.