Jicafold

I have a Trek 520 frame with Reynolds 531. Both of the seat stays are cracked right through the seatpost binder bolt. See photo. Its a nice frame so worth saving. I am trying to braze it with MAPP gas and it is not just getting hot enough to melt the brass. This isn't my first rodeo with brazing and have built of frame from scratch just to say I did it. However, I know some areas with more mass just require more heat. But I've brazed a bottom bracket before without issue. So why can't I get these seat stays hot enough and glowing orange to do this. Any ideas?

FBinNY 

The casting is a decent sized heat sink, and you may simply be lacking the BTU's needed to pump heat in faster than it flows away.

3alarmer 

.
...I'm not sure why you can't get enough heat from your torch to flow your brazing rod, but on a crack like that with no idea how long it's been busted (presumably a while), I'm not sure it's a good idea to just flow braze into the crack without opening it out a little wider and cleaning and fluxing the interior surfaces. Sometimes it sorta works, but there's doubtless some corrosion on the interior surfaces of the cracks that will often prevent a good, solid bond in the brazing operation.

Just an observation. If you could get it cleaned up, you might try a lower temp brazing compound like silver.

Andrew R Stewart 

I also suspect the torch is too little to best heat the work. MAPP is well known to be insufficient for brass work in anything thicker then tubes. I'll also agree that the proposed repairs are not likely to be long term. Lets see, big stress riser right where heat will concentrate during the initial brazing, the thinnest cross section around, bending stresses with clamping posts and Treks production liking fast brazing methods (as in a lot of heat, sometimes from more then one torch). This failure is no real surprise although it is the first that I've seen.


My thoughts about repair would be to replace the stays and binder barrel. While a fast back style of stay attachment (either to the binder barrel or to the seat tube below the lug) would preserve the "Trek" logo on the lug I would do a standard side of lug attachment. Simple, easy and avoids binder function hindrance.


But whatever is done ditch that MAPP unit. Andy.

dedhed

I'd send you this for the cost of a small flat rate box at USPS. From a 21" 1987 Trek 400 Elance with 531 main tubes. You'd have to check the Vintage Trek site to see if the frame angles were "close enough" to work.

Jicafold

I did temporarily separate the edges away from each other so I could clean / wire brush them as well as use flux. Still no dice. Andy, that is a good point about that being a high stress region, regardless of how well a braze could be done. Could easily break again. I see 4 possible options:

1. Use the piece offerred by dedhead, and thank you. My frame is a 24", but might be close enough. Still would need good heat though. Could use my buddies oxy/acetelyne torch.
2. Replace the stays as Andy says. I have some off another frame I could use.
3. Weld? Is this an option? I can always file off the excess and redrill the hole if necessary.
4. Make a lamp out of it.

Feedback please.

3alarmer 

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1. Welding is not an option, the bleed heat will transmogrify your 531 tubing.

2. With an ox/ace brazing torch, you can put out enough heat to do whatever you want on there. You just need to be careful not to over heat because of the aforementioned transmogification.

3. You can deconstruct, reassemble, pin, and forge braze the thing. But it's way too much work. It's very controllable in terms of the heating, though. And you don't suffer from localization in your heating attempts.

Jicafold

I could also use this piece off of a Lotus I have, same frame size. But I would have to "sleeve" the stays somewhere to maintain length. Using the piece from dedhead would make much more sense.

Dedhed. please let me get with my buddy with the torch and I will get back to you. Thanks.

WizardOfBoz

I agree with Andy: brazing on an extremely small section, which happens to be a built-in stress concentrator (which is why it broke in the first place) is not a good option. I think you have to remove the lug and replace (or repair the lug with welding when off the bike). Dedhed's offer is pretty nice, if the angles work.

This is just a poorly designed lug. Putting the seat-tube clamp right through the seat stays? What were they thinking?

squirtdad

you may also want to see what they say in frame builders....but you will get similar input

davidad

If you have access to a GTAW (tig) rig it can be welded. The alloy content is low so a heat treatment isn't necessary.

dsaul

Another option would be to drill out the holes to a larger diameter and braze in a separate binder boss. If it were mine and I just wanted to rescue it, I would drill/mill out the holes(probably large enough to separate the stays from the lug). Then I would make a binder boss, on the lathe, that was long enough to reach across both sides and braze it in. I would then cut away the center section of the binder boss. You could do this without making your own binder boss, but you might have to do each side separately.

Andrew R Stewart 

This is a very neat method and now I consider it what I would attempt first. Good one, dsaul!


I'll add to this that a smaller through hole in the binder, which will require a smaller diameter binder bolt, would add material and increase strength. I suspect the current binder through hole is sized for an 8mm diameter bolt (like a classic Campy or Sugino allen keyed one). But these bolts really have a M6 threaded section. So making the new sleeve with a M6 through hole and flat end surfaces then using a common M6 bolt and nut (or a fancy lathe turned nut) would result in no loss of bolt strength yet an increase of material around the crack. Andy.

dsaul

Thanks, Andy. Every once in a while, I have a good idea.

Jicafold

Those all seem like good ideas. I guess it depends on how much effort I want to put into this, but how often do you get a nice complete Trek 520 with a Reynolds 531 frame for $60? I'll have to go by my buddies auto body shop and check out his equipment as well as take up that offer from dedhed. Dedhed...PM sent.

mrrabbit 

Use MAPP with a silver brazing rod, not brass.

=8-)

Jicafold

That only melts the brazing material easier, not the joint. You still have to have the joint glowing orange hot for either brass or silver to properly fill the joint with "capillary action" and all that good stuff. I have the silver brazing rod. It makes no difference.

mrrabbit 

In your case we're looking at a lateral crack, not a sleeved/lug situation.

So it wouldn't hold irregardless using silver.

However, in a lugged situation, silver is what you'd use with MAPP.

Read a manual for a MAPP setup kit - it will tell you why silver works with MAPP.

Hint: It's not temperature...it's another property.

=8-)

ramzilla

Dude, no way is brazing going to fix that. You must be kidding me. That lug is toast. Need to replace it. Or, start looking for a replacement frame.

FBOATSB