Only reason to chase threads is if there’s a lot of rust. Mine weren’t too bad, I just sprayed with brake cleaner on exhaust threads and used a wire brush, and dabbed on plenty of antiseize on the sensor threads, screwed in fine.
You can buy an o2 socket at a parts/tools store, thats where i got mine. $15-20 tops?
Looks like this. I got 3/8" drive.
I’m interested to see how this turns out.
Hate to tell you this, but, if you had just wanted to tackle the valve stem seals, I have learned on here that there are a couple of easy ways to do it w/o pulling the head. One involves using compressed air through the spark plug holes to hold the valves closed (i.e. “up”) while you r/r each seal, one at a time. Another involves using a small length of thin nylon rope through the plug holes - crank the engine (pull the fuel pump relay or some such so it won’t start accidentally) and feed the rope in the hole until the rope has been squashed up against the valve, forcing it closed (“up”) … I admit the latter method makes me a bit nervous 
but so does the former unless I have a fool-proof compressed air source for the job.
I’m still stunned that when Jag first put out the AJ6 engine it didn’t even have any exhaust valve stem seals in the design. I don’t know why they thought they wouldn’t be needed. 
IIRC, it was part way into the '94 MY that the idea suddenly came to them 
I suspected that Superblue was produced just before that critical engine # (i.e. b/c of her blue smoke issue), but then when I checked her engine # against the changeover list I saw she was apparently one of the first ones fitted with the stem seals. 
My '95 4.0 XJS, Supercat, never had such issues though, and the mileage on her at the time she was totaled in her crash was about 2x that of Superblue when I first bought her. Makes me wonder if Jag started using an “upgraded” or better-quality stem seal by that MY. 
This is the actual wrench that A-Z loans out … Looks like it will reach it from above?
And this is their loaner tool “set” for O2 sensor. Note the “crow’s foot” configuration …
So which of the two tools would probably be more suitable for getting the job done “from above”? 
I’ve never seen a car you could do it from above. O2 sensors are either right before or right after catalytic converter. And that is usually tucked underneath the car.
Sounds like it is possible, though, Gregmatic, based on John’s experience …
Me too, VK … will keep ya’ll posted of what happens next, if it does … 
Well, this evening, with the engine fully cooled off, I decided to check out the location and appearance of the existing O2 sensor. Wow - yep, it’s WAY down there, but on top of the pipe. I could barely manage to squeeze my (right) arm all the way down to it and touch it with my fingers. I really don’t see though how it would be made any more accessible, even if on a lift, as the body bulkhead extends along the outer side of it at close quarters, blocking access from that angle (apparently). Frankly, I don’t see how it could be made more accessible other than by removing the catalytic converter – at least that’s how it appears from topside.
Anyway, I’m thinking the long-handled, 45-degree angled, version of the AZ loan-a-tool sensor wrench should still be able to reach it from above. The only thing is how to start the new one in the threads, assuming the wrench is not magnetic and strong enough to hold it while starting it. I THINK I should be able to start it with my fingers, but it will be a stretch (no pun intended). I’m worried though that if I can’t get the new one in and properly tightened then exhaust gases leaking from the mounting hole might cause some (high heat) damage in that area. 
Something odd about the OEM wiring I noticed is that down near the sensor the wiring has some kind of flaplike piece of (insulation) that slides up and down the wiring. The new Bosch sensor’s wiring doesn’t have that feature included. IIRC, Superblack’s power steering lines have one kinda like that as well. I guess the idea is it helps insulate the wires/lines further and keeps them away from touching any of the hot surfaces in that vicinity. ? Hopefully by sliding it off the old wiring and onto the new wiring I can reuse this one with the new sensor. ?
Sounds like finally something is simpler on the V12? 
Although we have two, they are conveniently located near end of downpipe facing outwards.
Someone with the straight six needs to chime in…there’s gotta be a simple way.
The AJ16 has four sensors. All easily accessible….maybe in the blind, but accessible nonetheless. I can’t imagine where this sensor would be on this AJ6 engine to make it so difficult.
Perhaps if you got under it, even though blocked from view, you could get easy access to it. Now that you know where it is, working by feel shouldn’t make it too difficult a job.
btw, I wondered why the little plastic cap on the end of the new sensor. Taking it of, the sensor appears to have some kind of black graphite powder on it, or some such substance … So are they “pre-lubed” that way with anti-seize or is that an anti-rust precaution?
Not sure, I always put antiseize on the threads, cannot hurt.