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I am working to restore my first mountain bike frame, a "vintage" 1993 Bridgestone MB-3. Early on, I swedged the left seat stay so that it buckled a bit & rotated about 15 degrees inward. I have sent it to a welder and am trying to source a replacement seat stay. Any suggestions?
Generic photo of the bike type added by Mσᶎ:
First "a welder" is possibly not the best choice if you have any alternative. If you can find a framebuilder, or a TIG specialist who deals with thin material that will be much better. If nothing else, you need to know that those frames are brazed together (you can see from the lugs in the picture), not welded, and if the welder tries to weld over the brazing the brass will boil off spraying metal everywhere.
The simple, brutal approach is just to cut the seatstay out of another bike and use that.
It's also tricky to weld thin bicycle tubing, so unless your welder has experience of thinner than 1mm tubes they're likely to make big, ugly welds and also damage the bike while discovering that. Blowing through the seat tube is likely to make the frame unusable, because even after you file the inside of the tube clean it's likely that the seatpost will either not go in, or won't clamp tightly.
That said, it can be done, but you really want a TIG/GTAW welder rather than MIG, and doing it with a stick welder is very, very difficult. I TIG weld my frames, and while I know people who MIG weld frames they do it full time and it usually takes an experienced welder a month or so (full time) to become competent at welding thin bicycle tubing.
The replacement seat stay is pretty easy to get. You can either buy a specific seat stay part from any frame parts supplier, or just use 4130 CroMo steel straight tubing that matches the thick part of the seatstay (probably 12mm x 0.9). If you told us what country you're in it might be possible to suggest suppliers.
You will want to recover and reuse the brake boss, and ideally also the rack mount at the top so remember when moving the bent tube that you're grinding the tube off the boss, not the other way round.
I have found that reversing the damage,if done right, can leave you with a near perfect and cheaply repaired frame. Park makes a frame bending tool. Look for it on ebay. If you put the hook at the most bent part of the stay, and if you put the bracing part of the tool at the right location, you can then pull out the bend without creating a compound bend. Unbending frames is an art. The more you practice the better you get. Try using the tool on a garbage frame until you get the hang of it. As long as there is no fracture or crease in the tubing there should be no safety problems with this fix. This is how I would fix a frame--for whatever it is worth.
