On 1 Aug 2018 21:25:57 GMT, Paul in Houston TX wrote:

IF it were my car and IF the manifold is steel, I would remove the manifold,
then remove the square nut, and weld or braze in a thin-ish nut of appropriate
thread size.
If no thin-ish nut is available then make one with the dremel.
The sensor needs to be in the flame and also have good thermal conductivity to
the manifold.

I had not thought of putting a new nut on.
I'll look in the morning to see if the nut is the only threads.
I'm sure the sensor has to be "in" the stream of hot vapors.

It seems to be cast iron. Aren't they all that way?
(It's rusty.)