Originally Posted by
unterhausen
the OP should really take the fork and headset out and look to make sure the crack hasn't propagated further than just the lug. I am in the process of fixing a bike that had a huge crack in the head tube. I was questioning if it really was cracked when I first looked at it, but then when I looked harder, it was quite large. In fact, the entire outline of the lug was cracked.
This crack will continue to grow
I'd recommend against removing the headset cup from the frame. If the crack is caused by pressure from the press-fit headset cup (as most people in this thread think), removal and reinsertion will cause a lot more stress to that lug than leaving it as-is.
Just ride it and keep an eye on it from time to time. It's not something that will fail catastrophically.
Your choice if you want to put in the money for braze repair somewhere down the line (if it needs it, which may never actually happen).
However, I have a sneaking suspicion that the crack is unrelated to the headset cup. The picture isn't very good so this is hard to tell, but the crack seems to be only on the exterior surface of the lug, and starts from the bottom of the lug, not from the top. So it may be unrelated and not a worry at all.
A better picture might give me a different opinion. Do you have a camera with macro (close-up focus) capability?