you can get weld on bosses for the lambda sensor but i always seem to have grief with them. i first drill a 20mm hole for the lambda sensor (M18 type wb NTK), then place the boss (with bung screwed in) over the hole and tack it in place. then weld around the rest of the hole slowly doing opposite sides to try to not put too much heat and distortion into it.

but more often than not, the boss does get distorted by the heat of welding it, and the bung won't come out

any tips?