Thanks for the info....
After some digging, I did confirm the thread size to be 18mm x 1.5.
Also found the Heli-Coil repair kit for O2 sensors, but the average cost is
about $100! That's a bit steep for a one-time repair! Looked for a place
that might rent the kit, but had no luck. I did find a 18.5x1.5mm tap at
work, and I lucked-out on this one... Looks like the old oxygen sensor took
most of the damage. The exhaust manifold threads were a little beat-up at
the top, but once I got the tap started, all went well.
This O2 sensor replacement took a lot longer than I expected, but at least I
didn't have to drop the $100 for the repair kit!