Originally Posted by
unterhausen
It's really difficult to ride a bike hard and long enough to crack in this particular location. Cracks happen near a stress riser, generally at the edge of a weld. If this was a fatigue crack, it probably would have started much nearer to that cable guide. I generally expect fatigue cracks on the seat tube joints. Ether on the top of the seat tube/top tube joint or at the bb shell. I think that there has to be a problem with a weld to get a crack at the head tube/down tube joint.
Of course, there always could be material defects. Some brands were known for it bitd.
It looks like it's cracked where the butting ends. Maybe a manufacturing problem-- the mandrel left some scratches on the inside, or the wall thickness wasn't uniform and too thin in some places? But also it's very rusty.