Is it a crack?
 

I've got an older Bianchi MTB frame made w/Columbus tubing; a real nice VERY light Chromoly frame. I plan on setting it up w/corrected rigid fork and a bunch of NOS shimano xt/xtr 8 speed stuff I have lying around.

Before I started stripping off the old components I noticed a bit of rust on the seam of the top tube to the lug to the headset. After scraping away the paint/rust in the area it looks like there may be a (tiny) bit of separation on a portion of the lug and/or the original braze was not complete for ca. 15% of the circumfrence of the lug/tube. Is there an easy way to tell if this is the start of a crack/separation? ( i was thinking perhaps magnafluxing?) Should I have someone rebraze the lug/tube? Any other suggestions on similar repairs will be appreciated

Thanks in advance, happy New Year

Tony

Only way to get an opinion that does you any REAL good is to take it to a frame builder.

You mention Magnaflux. IF you have access to it and a good skilled reader(the results can be confusing) then do so. The other method that is a little more available in the avation feild(and automotive) is Dye Penetrant, aka Zyglow.
I think you should be pretty well off by merely cleaning it thoroughly with a spray can of Brakleen, letting it dry and then examine with a 10x or so magnifier under bright light. If it is questionable, have someone with weld knowledge check it. Brazing is a very tolerant form of attachment, lugging is all the more. If it is separating(which braze generally doesn't) it is a simple matter of rebrazing it, which may not even involve more than reheating it.