I am getting ready to replace the top tube on my touring bike. I think it is rusting through, but I will do some investigating when I strip off some of the powder coat. Here is how I envision the process.
- Set up frame jig to fit frame.
- Cut the top tube in half.
- Remove the head tube from the downtube by pulling the downtube from its socket of the lug.
- Remove both halves of the top tube.
- Miter a new top tube. Braze in internal cable run.
- Refit head tube and top tube back on bike.
- Replace bike in frame jig.
I'd scribe a mark on either side of where I was going to cut, say 3". Use scribes to create your replacement top tube of same length (just to make sure you don't loose track of the saw kerf). Cut as mentioned. Totaly remove the top tube first. Grind crap out of lugs to ease fit. Then I would see if I could weedle the top tube in without having to remove the HT. The lugs aren't always that deep. If that proves impossible, proceed to plan B.
Also, the top tube is pretty much the last tube I would expect to rust out. I would wonder what else was rusted out. I guess you will find out.
I'm not a repair expert but I'd cut off the top tube leaving six inches or so sticking out of each lug. Grind the paint off of the lugs and look for evidence of pins (nails) used during assembly. If you find any, drill them out. Heat the tube and lug and try to pull the tube stub out of the lug socket - this is much easier if silver was used to assemble the bike in the first place. If you get lucky and the tube pulls out clean, a new top tube can usually be fit by springing the frame so to speak and slipping in a new tube. Of course, the inside of the lugs will have to be cleaned up first - a dremmel tool or similar would help.
Regarding use a a jig, most builders don't use one to hold the frame during brazing (other than tacking the frame together). The jig (fixture is the proper term) is to assure everything is in alignment prior to brazing which it typically done freehand.
Becareful buying/selling bike parts on-line. I learned the hard way. :(
I have done it twice, one was a 62 cm Trek road frame, it was easy enough to spread the frame and get the top tube in and out as Nessy mentions above
The second one was an early lugged 54 cm Specialized Stumpjumper frame, it being a short MTB frame with heavier gauge tubing I couldn't spread more than 1/2 an inch before I chickened out.
So what I did on the Stumpjumper was first cut out a 3" chunk of the top tube and remove the top tube from the seat and top head lugs, Next I heated the top head lug and pulled the lug off the head tube. After the new top tube was fitted and everything was cleaned up and ready to reinstall , I first slipped the top head lug over the end of the top tube. I then slipped the top tube first into the seat lug socket and then slipped the top head lug over the head tube. If I ever do it again I will definitely do it this way. Besides not having to spread the frame it also makes it easier to fit the top tube since you can see the fit of the miter against the head tube.
Thanks. I hadn't thought about trying to spread the frame to reinsert the top tube. I liked all of your ideas. I am not sure that it is rusting, but some strange things are going on. I built the frame about 15 years ago or so. It is a Tange Prestige tube set, silver brazed. I refit it two years ago into a touring bike. I replaced the chain & seat stays. When I opened up everything there was a bunch of brown chips & dust collecting in the bottom bracket. I couldn't tell what it was. I think that I checked to see if it was magnetic, is rust attracted to a magnet? but it was not. I knocked out as much as I could. But it would reappear every time I took out the bottom bracket. I can't remember if I put something in the tubes as a rust inhibitor that could be flaking off, but maybe. On to the next weird problem. I had the bike down to bare steel while refitting it. One night I left it in the bike clamp with a shop rag wrapped around it. In the morning that spot, a 5" section of the top tube was rusted and had several pits. I sanded it off. It rusted. I etched it, cleaned it, put on WD40. It rusted. Strange. Finally I took it to powder coating. I came back with the pits showing, but not too bad. However, in the course of two years the section under the powder coat is getting worse. it looks like bubbles or ? i am getting ready to pull off all of the parts, and will see what in the bottom bracket. Oh, that reminds me. The last time I took out the bottom bracket, which is a Phil Wood with a silver colored shell. The shell was pitted!!! I think my poor frame is dissolving!
Anyway, I up for replacing the top tube, and if that doesn't cut it, make a new frame.