Factory used to put them in new heads if they were needed.
Too much machining at that point to scrap the part.
Stock heli coils were used, but the heli coil had to be cut down to fit.
I want to say 2 threads cut off, but not sure.
Here is a D piston at top dead center if it will help.
Yes. Make sure you get an actually plug repair kit. I put one in one of my D series last year, when the brass plug adapter on my compression tester, decided to lodge itself in the hole. Jacked up the adapter and plug threads too! Not really happy about that, but the plug insert coil worked out real nice. After I ripped on the gauge manufacturer(Lisle) about what happened, I got a nice new adapter as well.