I'm not sure if you still have a question, but here is what I did on my '98 5.4, with 76k on it. A year ago I blew out number 2 spark plug. I bought the Time-Sert kit and did the repair myself. A couple weeks ago, number 6 plug blew out. I then decided to install inserts in all the remaining cylinders, which I did. The plugs closest to the firewall were a b***h!, but it's done. Note that this repair is not a helicoil, which is not recommended for this application. The Time-sert is a steel insert that you put in after cutting out the old threads (what's left of them) then reaming the hole with a "step reamer," rethreading the hole to accomodate the insert, install the insert (per instructions) and then the plug. In all the threads on the internet, I have not heard of any of these inserts blowing out. Afte 2 "blow outs," I had no confidence in the truck, which I now do. Good luck. Feel free to contact me if you need more info.

 

This past Friday, I had the #3 plug blow out on my '99 F-150 (5.4L, 179,000 miles).
I went down to the local auto parts store and bought a Helicoil spark plug kit.
It came with a tap, setter tool and 3 different size helicoils.
My brother and I did the repair in my driveway and even with taking our time, I dont think it took more than 45 minutes.
We simple fished the oil sparkplug out with a magnet and tapped the hole. Then, to get rid of the metal shavings, before we put the helicoil in, I started the truck and let it run for a second or 2. That blew the meal shavings out of the engine. We then installed the helicoil my putting it on a sparkplug and threading it in that way. You are supposed to use the setter tool, but we couldnt get the helicoil and the sparkplug to seperate with the coil in the engine, so my brother just tightened it up as hard as he could with a 6" ratchet, let it sit overnight to let the high-temp sealant dry and when I started the truck today it seemed fine.
We'll see how it goes. Im a little bit concerned that we didnt use the setter tool to mushroom out the top of the helicoil, but I dont see any way to get the plug and the helicoil apart while still leaving the helicoil in the engine, so I think we did the best we could. My father and my neibor are both Ford techs and they dont think that helicoil is going to come out, so Im hoping for the best.
Hopefully it will hold together and I guess at least if it blows another sparkplug I now have the kit, so we can do the job again.
Doing the job really wasnt all that difficult. Its kind of intimidating at first, but once you get into it, its not all that bad.
I dont think I want to have to do this job on the #4 and #8 cylinders (the ones closest to the firewall), but the rest of the cylinders arent that hard to do it on.
Personally, I think the whole thing is BS. IMO, Ford should be offering a lifetime free repair of this problem. I know it would cost them a fortune, but it would be the right thing to do. Im lucky in that my truck went so long before it popped a plug, I really feel for people who have less than 100,000 miles on their trucks and have this happen.

 

Im pretty sure its 14mm. The helicoil kit that I have says thats its 14.1mm, so Im thinking the plugs then would have to be 14mm.

 

I think the problem is more in the cylinder heads than in the sparkplugs. From what I understand, there are only 2 sets of threads holding each sparkplug in.
From what I understand, it does. My father works for a Ford dealer and from what he has told me, theyve had quite a few trucks come in with this problem and they offer a helicoil as a low-cost repair option (as opposed to spending $1,500 for a new head). He told me that they are yet to have a truck come back with a blow-out Helicoil, so that would tell me that it works.
My truck now has a helicoil in it and I plan on running this truck into the ground (its got 179,000 on it now and Im planning on buying a Focus for my daily driver and keeping this truck as my beater/wood hauler/snow vehicle) because its not really worth anything. So, if the helicoil ever comes out, you guys will know about it.