Originally Posted by scarlson

It's likely irreparable and will likely grow. That said, if it were $free, I'd probably put some components on it and ride it a few times just for kicks (with routine inspection at every traffic light; electric blue sharpie on the cracked area to reveal any growth).

Likely irreparable: the lugs are cast aluminum, which (depending on the alloy) ranges from "can't be welded; falls apart in crumbly mess when you try" to "likely weldable but only by an expert". Regardless, you'd have to get the other tubes out for a repair, because the epoxy bonding can't take the heat that welding would bring, even if you were to use an extensive heat sink or submerge the rest of the frame underwater (which no sane welder would agree to do).

Likely will grow: just my intuition. It is a crack in a casting that is under tension due to the tapered threads on the chainstay ends which are screwed into that lug and likely force that crack open. That's why I think Vitus frames are better designed. They put the castings under compression and the tubes under tension at the joints.