M18x1.5 mm bung (female screw) that can be welded to the exhaust to mount a sensor.
IMPORTANT WARNING: We are upgrading this part to 1.4541 or 1.4301 or 1.4571 stainless steel. Traditionally it was black steel (strength grade 8), and latest shipment was zinc-plated which is quite unsuitable for welding (for longevity) and being phased out in favor of a reasonable alternative. You get inventory warning during checkout and we recommend NOT to order this part until this text is updated (if you do order, the zinc-plated might be disappointing)
This is the size of the Wideband LSU4 "WBO2" sensor (or other lambda sensor, eg narrowband sensor as well).