When my old 6.9 was barely 2-years old, everything about the glow-plug system was shot, plugs, relay, thermostat, controller, all kinds of expensive stuff.
I don't think the internet even existed way back then, so no source of information available.
Knowing absolutely nothing about what I was doing, but probably as well learned as anyone else in my area, I just had the guy at the local parts-store order me in a set of eight plugs(ended up having to get the rest of the stuff from Ford, but at first I thought I only needed the plugs).
The plugs he got for me were A-C-Delco.
I just recently removed the eight little boxes, along with the eight old dud plugs, from my glove-box and stored them away somewhere; if I happen across them, I will check the number on the box.
Many years/miles later, those same A-C plugs were still in that engine; and, when I pulled the old 6.9 out, many is the person that told me that mine was the quickest/easiest starting Ford diesel that they had ever witnessed.
All of that other stuff was also still doing it's job.
Once I got the old engine out where it could be seen, I discovered two of the plugs had the tops completely burned off, and their wires were just dangling; they must have not been needed, cause she would fire right up with no hesitation.
;p 
) to want to try them. For my time and money, it's just sooooo easy to go with a tried and trusted product.
