There are several types of Heli Coil.
The first type that was developed looked like a spring.
You drilled out the hole a little and then you threaded it and then you wound in the Heli Coil with a special tool.
For a simple repair - where you were not going to take it in and out all the time - it was good enough.
The second type was one where you just threaded the hole to a bigger size and let the tap remove the material. You then wound in an insert and had to have a special a special wrench that wound it in by the fingers that sticks out of the insert. Once it bottoms out - you tap the fingers into the hole and that is it.
With a couple of drops of RED Loctite - I will guarantee you that it will not come out as long as the head temp stays below 500*
As far as a diesel being better than a gasoline - good luck!
45 - 50* Below is not Diesel weather.
We had a Ford Truck in our fleet, it was a 1 ton with a diesel engine, dual wheels, we used it to haul a 28' trailer for work purposes.
None of the guys that drove it had any experience with diesels. They would fill it up before they brought it back. Once it came back, sometimes it sat for 2 to 3 weeks at a time. One day my brother went to take it on a trip to New York and he made it about 10 miles before the fuel started to gel up. He was afraid to stop, because once it shuts off - you are looking at a major repair bill. Replace all the fuel filters, a tow, rent a second truck, put it in a garage to thaw out.
He couldn't buy any diesel fuel conditioner where he was at and so he detoured to my house and we went to a store and bought 10 gallons of Kerosene. At - 15* outside, buying kerosene and then dumping two 5 gallon cans into the truck wasn't my idea of fun. We ran it 20 miles and then we put a can of conditioner - into it. Then we had to worry about if we cut it back too far and maybe the engine would over heat. How we got the 10 gallons of Kerosene into it - I don't know. We just kept dumping and shaking the truck until we got it all in.
Anyone that spends any amount of time in Alaska would tell you that the extreme conditions makes it a whole other ball game when it comes to automotive parts and how long they will last in the bitter cold. Steel will snap and aluminum is no better!
If your Ford Head was Aluminum - my guess would be that the Alloy that they made the head out of was not the right one for the application. BUT, you can weld the holes shut and thread again - and the new hole would be as good as or better than the original one. We do it to racing heads for our stock-car all the time.
The 2008 Ford trucks comes with heated tailgates - so your hands don't get cold - when you push it home!
All service and repair manuals tell you to let a vehicle sit for at least 8 hours before you try to remove the spark plugs in an aluminum head. The steel plug welds its self to the aluminum and when you try to take it out - you take plug and thread at the same time.
Because of the location of the port for the head, it makes it almost impossible to get a tap inside of the port to clean the threads. They do make a underside Metric tap to chase the threads for all heads - that a good engine shop - should have on hand. The reason it is undersize is because a new tap removes material. Everytime you run the tap through the thread you remove material, soon you won't have any material to hold the plug.
But since spark-plugs works on electricity, the better the contact to ground is the better the spark.
Before you install the plugs, you should coat the threads with Anti Seeze. I use C5- A Fel Pro
http://www.newmantools.com/felpro/c5a.htm
