Right, I'm after ideas what might have happened before I start pulling head of the car.
Cylinder closest to the windscreen seems to be failing. Depending on the time of the day and position of the moon, it will knock quite badly or not at all, it sound to me like one of the valves is knocking on the piston. Car will start with no issues whatsoever when cold, may or may not start knocking immediately but when it does, I can rev the engine for a few seconds and knocking goes away but I can hear what I would describe as the compression escaping, like pumping bike tyre with a hand pump.
It definitely sounds like its coming from the top end and from the back of the engine. Performance wise it seems absolutely fine, I can happily rev the bejesus out it but it does sound like its just about to fail.
What I cant understand is how come it randomly stops knocking when revved.
I think if I go the head off route, I'll probably replace all the chain guides, etc but ideally car needs to survive another month as is

 

The knock sensors can be swapped and see if there is incorrect spark plug timing issue

The knock sensors should retard the timing and you may be coming up on the point where it would retard the spark

The coils can be separately swapped also to see if it follows

There is a history of the aft 2 coil wires being swapped and there is a color code referencing the wire diagram

The aft most coil # 6 would be light green / purple and # 5 would be light green / orange

There is also the issue of the coil connector not being fully engaged and clipped

 

Nothing to do with knock sensors but the culprit has been found.

https://photos.app.goo.gl/z7gbsiL6nWbSuUHx8

At this point I was 75% its the valve knocking on the piston, 25% that its burnt valve and knock is basically a blowby into exhaust(which you can just about hear in the video).

I just couldn't understand how was that physically possible and how to explain it without taking head off.

(at this point in writing this post, I thought I'd go and check the cam cover before finishing this post...)

Took the top cover off, as the sound was a bit like the coil arcing to cam cover just much louder. I then undid the 6th coil and proceeded to start the engine. At this point the spark plug shot hiiiiiiiiiiiiiiiiiiiigh in the air and landed about 6 meters from the car! I suppose this is one of these life and death situation as I was definitely close to looking into that hole

Anyway, don't think I ever heard of anything even remotely similar to this situation with either AJ6 or AJ16 engines. Threads of course are gone.

I don't understand how was that physically possible, especially since the car ran absolutely fine, even with this knocking which would disappear completely within 50 meters of me driving it down the road, idle would settle and the car would run ok
Somehow spark plug must have been sealing itself to the head at load. I've replaced these 3 years ago, tightened to spec of course and I'm completely baffled now

Now need to do some research into time-serts, anyone here ever used them? Alternative is of course replacement head but ffs, this is really worst time for this, absolutely need the car in two weeks
I assume I'll need an M14 1.25 time sert. Any advice anyone?

 

it really wanted to be a ford so it ejected a spark plug 😪

use the heli-coil brand or some parts store rethreader kits. if you feel like it some retaining compound. jb weld, the many red loctites, or the green ones lots of options

 

Right. Its all fixed and back to running perfectly. Just took it for a spin, 10 miles of spirited driving and it ran absolutely fine. I've used a time-sert, M14, 1.25 pitch, 16.8mm long. I have also bought a Neilsen CT3417 spark plug thread repair kit, the kit was needed to properly fit the time-sert(it has a driver for it). All went in ok, two passes with taper and plug taps, then a reamer to clean the top and to make a little grove to sit the time-sert. Then screwed in time-sert with an install tool, it went in tight in there and of course I've used some loctite. I then cranked the engine to blow any swarf that got into cylinder and then tightened spark plug up to spec, put a spare coil in and all seems fine. Lets hope it lasts and I don't think I'll ever wanna take that spark plug again!

If it however fails, I will let you all know.

Time-sert definitely looked slightly thicker and looked better quality compared to heli-coils. It was 20GBP and a kit was 50GBP.