Now that you mention it:
I had really really dark grey stuff in my exhaust. My instructor was saying that it's just the engine breaking in, but some times it was really bad. The LHS said I was running too lean, but I know better then that. At the pattern meet I was told that running lean wouldn't do it - dark grey means aluminum; the ring and sleeve are NOT aluminum. I did find that the muffler baffle had been vibrating, and there was some slight wear on the edge of the baffle. Removing the baffle took care of MOST of the grey, but not all of it. The wear on the case is very slight, and extremely difficult to see, if any really, on the rod. I've been running the engine pretty rich since day one, so maybe it's not as bad as it could have been.
Maybe the extra heat and vibration from the rubbing caused the bearing to start to degrade.