theopowers

I posted this in Mechanics, and am reposting here on advice of another member:

So I am helping a friend build up an 80s-era diamondback road frame, and we have hit a snag - the frame is in good shape but we hadn't noticed that one of the threaded downtube shifter bosses is cracked. We are using STI levers and planned to use the usual screw-on...uh...thingees to run the derailleur cables through at the downtube bosses.

No damage to the frame or the brazing, just that the part that sticks out and is threaded is cracked. Is this an insurmountable problem or was there a common fix back when everybody had downtube shifters?

Sorry for no pics but I don't have the bike on hand.

Thanks!

John E

The quick-and-dirty solution is to use a clamp-on downtube boss. If the cracked boss will still securely accept a mounting screw, perhaps with a dab of LocTite, you may be able to use it as-is. Brazing is another option, at the risk of paint damage.

Otis

Hard to say without seeing the crack, but I would either: Go ahead and mount the STI cable stop using some JB-Weld or epoxy on the screw and around the braze-on. Modern derailleurs are pretty light action so I don't see why that would not hold up. Or, if the crack is so bad you can't even mount the cable stop I would saw off both braze-ons, file smooth, and use a clamp-on cable stop for bar-cons, or a clamp-on DT shifter boss with the STI stops in place of the shift levers. That's assuming you have an older bike with a normal diameter down-tube.

vpiuva

I had one fall off of an old raleigh technium that I just epoxied back on and it held for a DT shifter, so I would think JBW or epoxy should hold it in place for a cable stop.

theopowers

Thanks to all for the replies. We have a clamp-on setup available but I think it's ugly and would like to avoid using it. So we're going to give JB Weld a shot. I'm told you can actually tap and screw it, so we might give that a shot if we're feeling patient. Otherwise just glue the damn thing together.

theopowers

Well, here is my "fix" if any are interested. I didn't want to just glue it on because...uh...craftmanship...uh...future rebuild...uh, whatever. My idea was to reconstruct it with JB Weld, then re-tap the screw hole and see if the JB Weld would be strong enough to hold the threading.

Unfortunately I need to get this done for a friend and I didn't have the requisite metric tap or drill bit, so I ended up faking it just drilling it out with a fractional bit that seemed slightly larger than the metric bolt.

Aside: Where in the hell does one buy metric drill bits, taps, and hex sockets? Home Depot and Autozone were no help to me at all, and like I said, I didn't have time to order stuff online.

Anyway, I glopped JB Weld in and on. Ideally I would have liked to have used a syringe to pump it in and eliminate any air bubble, but since I didn't end up tapping for the screw, it actually worked out that I only got the weld about halfway in, because it may have left some of the deep threading intact.

That was Friday. Today I used the dremel to grind off the excess (much easier than I expected) and I drilled out the hole. Once again, ideally I would have set up some type of leveling or machine press device, but in a hurry I just eyeballed it. Took out a little chunk at the end but no worries.

Since the part was actually split open and widened, I had to grind down some metal as well, which was good too because the downtube stop is now a really tight fit.

Ultimately the bolt kind of screwed in, and kind of not. I can keep turning it all day and it won't stop, but it does back out when I unscrew, so there is some level of threading in there. We'll see how it holds up to road vibration.

theopowers

Now, does anybody know if I could use a thread locker with JB Weld? The Park Tool book says that thread lock can harden and crack plastic parts. I have no idea if this would apply to this stuff.