kroozer 

My 1992 Trek 520 (4130 chrome-moly MTB tubing) has a very slightly bent downtube. It’s on the underside, right behind the head lug, typical head-on collision damage. It’s a pretty slight bulge-- difficult to see and you can just barely feel the bump if you run your finger over it. How compromised is the structural integrity of this frame? Would you do a loaded tour with it?

This has been a great bike-- I commuted on it for 18 years and have certainly gotten my money's worth from it-- and I’d like to keep it going. On the other hand I don’t want to ride it if it’s no longer safe. Plus it needs a new fork (top of steerer tube sheared off).

ftwelder

It has been said that if you make a pair of hard wooden blocks that you can clamp them around the tube in a vise and twist the frame a bit you can remove small wrinkles. I have tried it and it works. On butted tubes, they kink at the end of the internal transition (about 120mm down the tube) or very close to the head tube in the thicker area, sometimes both. It sounds like your damage is very slight. I would prefer to see a clean side view photo of the bike to see exactly how much it have been altered before passing my opinion along as to it's structural integrity.