Yeah Gary, quite right. I just find it more convenient to regard the front of the car as the reference.
Frank, it sounds as if that was a different problem, mine had blown out completely.
Anyway, to get back to the solution. I decided that it was too big a job to remove the head and that it could be quite safely done
in situ. I had purchased this progressive tap-
and some inserts of which we chose the right length.-
We had an investigation with a bore scope to see what state everything was in and turned the engine over to make sure that the piston would not be in the way of the quite lengthy tap.
The big problem is that one cannot see so far down the hole to see what is going on, so with a big breath and a liberal application of grease on the tap to catch the swarf, we wound in the tap a few turns until it was hard going. Then the tap was withdrawn, cleaned, regreased and wound back into the hole.
Now, because of the progressive nature of the tap, with increasing diameter of the cutting edges, it is important to go deep enough to ensure that the insert will screw in to its full depth. There is only one shot at this, it's not possible to unscrew the insert once it's in position.
The big danger on our minds was that we would screw it too far and the tap would drop into the bore. So the in and out, clean and regrease procedure was carried out many times and we counted the turns to make sure that we had gone far enough.
Another inspection with the bore scope showed negligible debris and no big bits of swarf on top of the piston. To be sure, we started the engine for a second and any bits were blown out of the spark plug hole. It's only aluminium debris anyway so anything left will most likely get blown through the engine.
Now was the point of no return.
With some thread lock applied to the outer diameter of the insert it was screwed onto the plug and then wound into the head quite firmly. We left it for a few minutes while cups of tea were enjoyed and then wound the plug out. Very relieved we were to find the insert fitting perfectly in the hole.
Plug replaced, a new pencil coil connected and - moment of truth- fired up with no problem and no sound of leak-by.
Why did it happen?
I can only think that I had not tightened the plug sufficiently when I changed them. I most certainly did not over tighten them and I am the only one to ever change the plugs. (Very expensive special LPG
(Liquified Petroleum Gas) plugs they are too.)
A quick check of the other plugs showed that they were tight enough, so mind put at rest. Anyway, I have enough inserts to do this again, probably on the V8 as they are susceptible to blowing out.
)