Crack in lug and DT - fixable without replacing?
 

So I got this otherwise nice Colnago Super that has a crack around the HT lug and the DT.

https://i.postimg.cc/sD1LkkpB/IMG-5711.jpg

I am wondering whether a small hole at the end of the crack in the DT to stop it and brazing a filet on top of it all around the cracked area could be a permanent fix.

Does not need to be cosmetically perfect but I just don't feel like throwing the whole frame out.
It does not appear that this was caused by impact but rather stress and tension over time.

I don’t think a hole will work. It looks like the crack goes around the lug as well as from the lug onto the dt. That’s also a high stress area. I wouldn’t risk it

A hole will stop the crack propagation, but the tubing and lugs are the structural parts, brazing material is the glue. If the structure is cracked, not sure fixing it with a softer material will make it right. Lots of builders do tubing replacements, several post the costs on their website.

That's a very high stress area, so a fillet is unlikely to work. It looks like the tubes may not have good contact, with the lug carrying some of the load.

That was the point I was trying to make. The lug here is not supposed to be structural, it should really just serve to keep the head tube in contact with the downtube and allow the brass to bond them.

I don't think anyone stop drills a crack and doesn't do some other repair. I would be glad to be informed differently. And I'm pretty sure that stop drilling is considered ineffective anyway, at least that was said in the Air Force structural repair training that I received. The main reason being that nobody ever catches the true end of the crack. So most stop drill holes just weaken the structure. I used to get pestered by Navy engineers occasionally because the Navy doesn't allow cracks, no matter how they have been treated.

I will go further and say that stop drilling is a sheet metal repair technique and that no engineer would design a repair that included stop drilling something like a joint in a bike frame. At least not without some added structure.

Sorry - the down tube and lower head lug need to be replaced. If it were my work to do I'd replace the downtube, headtube and both head lugs. Faster and cheaper and a better repair in the end.

dave