The number three (of course it's in the back middle) spark plug blew out of the hole. It happened twice. The first time about three months ago I thought it was very strange. Maybe I forgot tighten the plug? I installed a new plug and tightened it carefully. It seemed fine. Now it blew out again. This time there are very few threads left and the plug just falls into the hole. I've read that this happens regularly on the Ford Triton V8. I can't find any info on the Chrysler 2.7L. The motor has about 80K miles.

The options are Heli-coil, Time-sert, and the worst is a new head. I haven't been able to track down much info...

Plug spec's are M14x1.25. The thread length is 25mm or just under 1". The only insert that I can find is a 0.75". Lisle tool rep says go ahead and use the 0.75" insert and it will hold well. There will be a small area of exposed thread around the bottom of the plug. Sounds like a plan.

Success, used a tapping tool that took the old threads out and then continues to cut the new insert thread in one step. First step was to jump the starter relay to advance the piston to compression stroke and then a little more. That makes sure the valves are closed when the chips are blown out after tapping. Two small extensions did the trick to turn the tap. The tap got stuck in the hole several times. A small hook tool caught the edge of the tap to pull it out of the hole. The tap tool threads were spray painted white to assure the proper depth of the cut. A small tube was used to spray air down the hole to blow a large amount of chips out of the hole. The insert was put in with thread lock. I will post a link for photos.

Another 100k miles and I may need to worry about the plug.

 

Spark plug threads can get buggered if you have a chunk of carbon deposited on the end of the thread and then back out the plug with that chunk there. The chunk gets dragged through the threads and makes a mess. The trick is to loosen the plug just a little, then run it back in to hopefully drop off any debris before it gets trapped in the thread. If it seem like it's coming out with difficulty, stop immediately and run it back in.
That short insert might increase the chances of getting deposits on the thread. Spark plugs are supposed to go 100,000 miles these days so maybe it'll be the next guy's problem by then.

 

The car will go to the junkyard with the new spark plug and the new insert :0 Stand by for the outcome of getting the tapping tool and extensions into that back spark plug hole with the cowling that restricts the access. I will try to get some pictures...

 

You can easily remove the rubber on the edge of the cowl for a little more room (not much). I found when installing plugs back there that putting a u-joint between two short extensions allowed me to warp the socket down into the hole. There was just enough space to swing the wrench.