What caused my spark plug failure?

So, I've almost completed my KZ440 project.
It runs, or at least, it did run. It requires a couple of electrical items to be renewed, but I'm almost there.

Today I was chasing down an issue with a grounding problem stopping the horn from working. I worked out the problem. After that, I decided to start the bike up again, just to confirm it worked as it did a couple of days ago. Heard a couple of misfires, then it was running on one cylinder. The cold cylinder was absolutely dead cold. So I took out the spark plug. It looked good. Tested it, and there was no spark. To be sure it wasn't the wire or plug cap, I put the spark plug from the other cylinder in, and tested that. That one was OK.

This surprised me, because both plugs are "NGK Iridium" (fitted by a previous owner). I didn't change them because they looked good. All I ever did with them was gap them.
So I looked up a video online, and read a few things. I think both of these plugs might be fake (counterfeit). I'm perhaps 75% sure. The thing is, the threads on these plugs look well cut, and they have a little bulge which looks like a welded iridium tip, which is what they're supposed to have. They have tooling marks (sort of lathing marks) near the hex, and I don't think they're supposed to have that. Plus the neg electrode is sort of right-angled (flat at the top) when I think it should be curved all the way. Even though they might be fake, actual iridium plugs are supposed to last forever (almost), aren't they?

I've got some normal plugs on their way to me now. But I wonder what everyone's thoughts are on this. Why might a spark plug work one day, and then fail permanently after misfiring a couple of times? I see no damage on the insulator, and the failed plug is clean.

Some may have other ideas, but I shudder a bit when I hear people say something is supposed to last forever.  Reminds me of the 100,000 mile spark plugs from GM.  They MAY last 100,000 miles, but good luck when you try to get them out of the head and take the threads with them due to carbon buildup.  Spark plugs are an expendable item easily replaced.  I would just replace the plugs and go from there.  Sometimes problems arise and there is no obvious or reasonable explanation for it other than replacement with a quality item.  Good luck and don't spend too much time trying to figure out what happened.  Just try new plugs.

Rick H.

If it was a cheap fake, then a sudden failure should be no surprise. Making a fake requires no quality control.

Even the best of companies sometimes let a sub-standard part get out to a customer. One of the reasons I prefer to shop in person is that returning a faulty product is usually pretty simple. Since those were installed by the po, I would buy some new, correct plugs from a real NGK or ND dealer and get on with life.