A few months back, towards the end of a short weekend tour, the BB on my trusty Trek 400 started creaking with each pedal stroke. Poking and prodding at the bike, I discovered what looked like a crack on the edge of the downtube-to-bottom-bracket lug:
I posted to the Framebuilders' List about this problem
, and while most agreed that it wasn't worth it to replace the down tube on this old, off-the-shelf frame, I got some recommendations to try to patch the area. So that's how I proceeded.
First I stripped the paint off the frame in the affected area. Then I cut and shaped a triangular patch out of an old chunk of fork steerer tube:
Next I brazed the patch on as best as I could, trying to make sure that brass filler fully penetrated between the patch and the frame tube, as well as building up a blob of brass across the patch and the lug to strengthen that area:
This was harder than I expected, because the BB shell really sucks up any heat you apply to the lug socket. You can see the black spots on the patch where the flux got overheated and scorched. In retrospect, I should've used the largest tip on my torch, instead of the same one I use for rack-building. However, I posted the results back to the Framebuilders' List, and got some encouraging comments that my repair looked fairly OK.
Then I sanded and primed the repaired area:
And then painted the area with white spraypaint:
The result looks a little blobby up close, but from 6 feet away, you'd never notice it.
I reassembled the bike with a new cartridge BB since the previous one was starting to feel a little notchy. So far, the bike's been as good as new, and the creak is gone! I've since ridden this bike on a couple of mixed-terrain rides, plus another weekend tour, and it seems to work as good as new.