Update. Cleaned the other spark plug holes and low and behold it's had this issue before. Surprise, surprise 52years! I can see one that has had no work as the top seating area is totally flat. Another one had a raised shoulder which I believe is an insert. The third looks like a sealing washer from an old spark plug which has rusted on. Strange if it is as it has little holes in it. Anyone recognise what it could be? My main question is if the new plugs could deal properly on the raised shoulder inserts?

 

The one with the holes, or dimples, looks like someone took a rounded end punch to spread the aluminum to keep the insert in place, or what ever is in there.

At any rate, this sort of thing happens from owners who over tighten the plugs with out any never seize on them.
The aluminum is easily galled and stripped against the steel spark plug threads when care isn't taken or from a red neck who thinks they have to be reefed on.

 

Thanks Jeff. The holes are too regular for what you suggested. I've tried 'pinging' it off but no joy. It does look like a layer of metal or thin washer. As I said, maybe an early, compressable spark plug seating washer.

 

Bit of a mess. Whatever you decide to do the plug protrusion into each cylinder should remain similar. The plug to head contact should remain good to conduct heat away from the plug into the head. i.e crush washers should ideally make proper & even contact.

 

Hi Glyn. Yes, totally agree. With this in mind I might try a seating tool that puts a very small chamfer on the top edge of the spark plug holes that have this small raised lip. Could be the top part of a helicoil that has not gone down far enough. What I did make which helped no end today we're tiny inserts. I cut the thread part off an old spark plug (1/4" or so) put a small slotted nut and bolt down the middle secured with loc tite. Then screwed in to just below the seating surface. Cleaned everything up, blew it clear with an airline and screwed it back out again.

 

Sounds like you are heading in the right direction!

 

At one time I owned a partnership interest in an independent shop that serviced Jaguars, Mercedes and Volvos. The rear spark plug hole on the Mercedes M110, straight 6, was notorious for being cross-threaded by ham handed mechanics. We repaired many. After using heli-coils for years, we finally settled on the Wurth "Time-Sert system for all cars with alloy cylinder heads, including Jaguars. Just one more opinion tossed into the hat...

 

Jeff. You were spot on with your suggestion that it was a punch used to spread the aluminium. 3 of the plugs were not sealing. I’ve taken all the high points off of the helicoils and flattened the sealing surface. All works a treat. Rethreaded the stripped plug hole so well please with my days work. Have to keep tapping Rev counter glass as needle sticking. On to the next problem

 

Good news!