Time to say goodbye ?

The how to not bugger up your spark plug threads is here: https://www.the-t-bar.com/forum/tooltime/99342-how-not-to-bugger-up-your-spark-plug-threads

I didn't want to spoil Cobber's excellent thread on helicoiling a thread so I will leave this here.
Cobber alluded to swarf getting in to the bores and it can't really be avoided but you can minimise the risks with string.
Grease up some thinnish string and feed it down the plug hole. Let it sit on top of the piston, it can't go anywhere, raise the piston up close to TDC so it is high in the bore. That way any swarf can't spray outwards it can only drop downwards. Removing the string brings out the swarf..or most of it anyway.

Another use for string, especially in pushrod engines like the B's etc is to use it to shore up a valve.
If a valve spring breaks or you need to change a valve stem oil seal you can do it without removong the head.
Put the piston about halfway up the bore, drop a pile of string down the plug hole leaving a length showing to be able to pull it back out.
Raise the piston and the string will compress in to the combustion chamber supporting the valve.
You can then remove the valve spring retainer and the collets and the string will stop the  valve dropping.
If the valve isn't supported it makes it impossible to compress the new spring and fit the retainer.
You sometimes have to tweak the crank to force the string against the valve but it does no harm. Once done remove the string and carry on as normal.

We used the 2TB pushrod engine a lot which revved to 8k. It broke valve springs. This saved removing the head.

Okay so my helicoil kit came today with tap + inserts.

I first made sure the piston wasn't too high in the cylinder that the tap would hit it. I greased up the tap, clipped it into the long extension, got it into the spark plug thread and screwed the tapered guide in by hand. I then attached a ratchet and as cobbs guide suggests, slowly tapped 3 rotations forward and 2 back which was a success. I then screwed my insert onto the spark plug, screwed in and it the good news is that I have a nice new strong thread to work with.

I could have left the spark plug there as it would have been flush but I wanted to get the job done properly. The insert I'm using has outward teeth at the top of the insert, on the outside and above the thread which are designed to be lined up flush and punched with a tool provided with the kit to expand the teeth into the block. Preventing the insert from exiting the thread when you take the spark plugs out in future.

 

 

 

When I first took the spark plug out to inspect, the insert came out with it. So I thought I'd have to screw it in nice and tight for it to stay put. I took the spark plug out again and noticed that the insert was too high and not flush so I put the spark plug back in to screw down again. I've then somehow managed to screw the insert too far down into the thread, so the thread on the spark plug won't catch it.

On the bright side, at least it hasn't fallen into the cylinder because that would be a serious pain. At the moment, I need to figure out how I'm going to grip that insert and raise it up the thread without that happening.

I would like to punch the insert in like it's designed for. But I also can't think for how I'm going to get the insert flush and free in order to do that.

Any suggestions gents ?

I'm not familiar with that type of insert, the other type I briefly mentioned  have tapered strakes that are used to screw in the insert using a special tool  that once the insert has been screwed in the tool is turned unside down and is stuck by a hammer to drive the strakes in to the hole around the insert to permanently lock the insert in place.
Here are some pictured
 
I'm not aware of these being available for the repair of spark plug threads, I would imagine the awkward location of many spark plugs would make these type of inserts too difficult to install.

The type of thread insert I was referring to are a helically wound length of wire with a diamond shaped cross section.

 

From what I gather from what you've written here you should've locked it into place by punching it before you screwed in the plug and moved it!
Looking at the insert  in the pic were it's sitting on the on the plug I can see it is knurled above the thread. I guess that when punched this knurled section is meant to deform to lock the insert in place.
What is the brand of insert?
And how far is it too far screwed in?
I'm sure we can come up with a recovery method. However that particular insert may not be reusable once we get it out, you may have to use a fresh one, it all depends on how hard it is to recover.
 

The brand I used was Sav-A-Thread > https://www.amazon.co.uk/Helicoil-5334-14-Thread-Thread-Repair

It seemed to be the only one I could find online with evidence that it could be installed without taking the head off. I think if the head was taken off, it would be easy to line the insert up flush and lock it in with the punch tool. But doing it without, with an extension that far down into the engine, the only way you can guarantee it's really flush is by screwing it in with the spark plug.

I managed to get the insert back out with some gaffer wrapped round an extension in the end. The insert was good as new so got it back on the spark plug before screwing back in. Prior to this, I fashioned some long pvc clear flexi hose from a brake bleed kit onto a vacuum attachment to get in and clear out any remaining swarf.

I removed the lower rubber portion of the HT leads and cleaned them up before putting it all back together. I then installed the new downpipe onto new previously fitted backbox & cat. I used a thin layer of firegum on the flanges along with the gasket on the cat end with gasket only on the manifold end as it says in the how to (to prevent damage to o2 sensor).

Got the car flat and checked the oil level which was at minimum so topped up with half a litre I had from last oil change which brought it up to mid. On road test the car runs and sounds better than it ever has with an increase in power. I was so happy. It hasn't felt that good since I've had it. The EML light is still on but I imagine I just need the codes cleared to reset it. A local guy I've used before has a diag box. I'll report back following.

One thing I'm scratching my head about is the oil down both sides of the engine from the cam cover. When I had the gasket out there was zero damage present. Beyond that the only thing I could think of it being was a torque issue.

I plan on doing oil & filter this weekend. I'll clean up the engine when I do and check ongoing if the issue has been rectified.

All's well then, you got the bugger out with out drastic measures then.

Well done! Pleased it's all coming together for you.