Here's what I am looking at.
The dark ring varies in height. In one spot, the shiny plating goes almost to the top of the bore.
In another spot, the dark ring extends down what appears to be 1/4".
The height variation of the dark ring cannot be explained by honing. If the hone stopped short of the top, then the affected area would be a consistent height.
An inspection mirror helps, but it's still not the same as looking at the area directly. That requires either a borescope, or sawing the jug in half.
The dark ring appears to extend down far enough to be swept by the rings. If the dark ring is indeed unplated, then it'll still run, but there will be questions about ring sealing and longevity.
Here's one of the jugs that I sawed in half. Before sawing, I could only see a mysterious dark ring near the top of the bore, and I could see that the ring varied in height, similar to your mysterious dark ring. After sawing, it was clear that the ring was partly due to a huge gouge. The variation in height was due to missing plating, not missing honing. My guess is that this area is "sunken" due to a casting defect.
The grey stuff is bare aluminum. There are no honing marks, because there is no plating. In any event, this is not conducive to sealing the rings.
I am not familiar with the plating process, but apparently any spot that is greater than bore diameter does not receive plating. For example, if there is a gouge, the gouge does not get plated. If the casting has "sunken" and pulled away from the bore, the "sunken" spots don't get plated. The area around the decomp hole often seems to be missing plating, perhaps because that spot is a little "sunken."
I'd love to hear someone who knows more about the plating of chainsaw jugs -- hint, hint, Timberwolf, if you are reading this
-- explain what kind of things go wrong that cause the plating to be screwed up at the top of a jug.
