Here's the post-mortem on the front cylinder that seized on stage 4 of the 2018 Cannonball, then ran for 3,000 miles after that. As you would expect, the piston was pretty well cooked.

The cylinder had some scoring, but mostly it's just aluminum stuck to the cylinder wall. It would probably hone out and be useable.

And here's the smoking gun. A crack had formed between the threaded holes for the valve plugs. This caused a vacuum leak, especially once the engine warmed up and the crack opened up further.

This kind of cracking has been known to happen on these cylinders, usually believed to be caused by the original bronze valve plugs that expand at a different rate than the cast iron. I don't use the bronze plugs, but the cylinders did have bronze plugs in them when I got them.

After the crack formed, the vacuum leak caused that cylinder to run lean and hot, and it seized. It seemed to happen all of sudden, not gradually. The engine was making really good power, then all of a sudden it was not, then it seized in a very short time. After I got it running again, I could see by the spark plugs that the front cylinder was running leaner than the rear. In fact, to get the front plug to read right I had to richen up the carb to the point that the rear plug would get black and sooty. It was a pretty clear indication of a vacuum leak on the front cylinder, but I wasn't able to find the leak with the equipment I had with me. It clearly was not at the intake manifold. A pressure test would have found it, if I could have done that. In the end, what I did to get me through the Cannonball was I went to a tractor supply store and found a hotter plug for the rear cylinder so it wouldn't foul, and ran the carb rich enough that the front plug looked right. You do the best you can with what you've got.

I'll probably braze that crack up, hone the cylinder out, and use it as a spare. It will be a tedious process due to the fine threads, but it's worth saving the cylinder.

I'll try to get some pictures of my intake cam followers before I go on the road again. I'm really happy with the way they performed.