Arlen Holder wrote:
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.

I wouldn't worry about it. Any leakage would be minor.
However, if you are worried about it you can put some high temp
sealer around it.
Don't know if a hose would work in this situation or not due to engine noise.
Get lenght of small hose, stick one end in ear, and use the other end
to listen for leaks.