On 3 Aug 2018 18:03:11 GMT, Clare Snyder wrote:

http://www.bild.me/bild.php?file=3408929o2sensor13.jpg
http://www.bild.me/bild.php?file=8568761o2sensor14.jpg
http://www.bild.me/bild.php?file=5285317o2sensor15.jpg
Thanks for reposting. Yes, it is a fabricated manifold. What metal
it is is hard to say - likely a high nickelteel - similar to
Stainless.

Thanks Clare for your always wise and wonderful advice.

I have a friend with welding equipment lined up if I need to drill it out
to insert a bung and have it tack welded in place.

At the moment, I'm sticking with the simple plan of...
1. Hope the thread chaser worked (how would I know if it didn't work?)
2. If needed, try the M20x1.5 tap to add a M18x1.5 insert
3. If really needed, drill it out to have a bung tack welded in place

I think, as you astutely predicted early on, that #1 will suffice.

My main mental dilemma is only how to tell if gases are leaking past the
lambda sensor threads under pressure.