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Cracked toptube/seatube lug

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Cracked toptube/seatube lug

Old 11-17-23, 01:46 PM
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Cracked toptube/seatube lug

Do you have thoughts on repairing the cracks in this frame? I think it will be fine to pinch the lug to bring it back into shape, then heat the lug and drop in a bit of brass. Then clean it up with filling and repaint. Thoughts? Would you aviod repairing these cracks or use a different method?

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Old 11-17-23, 08:03 PM
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They look like someone cut them with a hacksaw and tried to fill them with some body filler or similar material. Personally, I would clean them up, TIG weld and file them smooth.
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Old 11-18-23, 05:36 AM
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Originally Posted by dsaul
They look like someone cut them with a hacksaw and tried to fill them with some body filler or similar material. Personally, I would clean them up, TIG weld and file them smooth.
Yes. And would probably need to run a reamer down there after welding.
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Old 11-18-23, 12:49 PM
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I have heard that tig and brazing don't mix. Two tig welders disagree?
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Old 11-19-23, 05:59 AM
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Originally Posted by unterhausen
I have heard that tig and brazing don't mix. Two tig welders disagree?
For something like that, I don't think a little bronze or silver would hurt anything. It's also possible that the seat tube doesn't come all the way to the top of the lug.

On a somewhat related note: I just found that I can use my TIG torch as a heat source for silver brazing on a recent repair. I welded a chainstay yoke to a fillet brazed frame to replace a broken chainstay. I couldn't weld the spot where the yoke intersected the seat tube/bottom bracket fillet and tried to silver braze it with my MAPP torch(I'm out of propane and my O2 concentrator is acting up). I wasn't able to get it hot enough to flow the silver, so I cleaned off the flux and used the TIG torch to melt and flow the silver filler into the joint. It worked surprisingly well. Time will tell if it made a good joint, but it was mostly cosmetic, since it was welded along the bottom and sides.
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Old 11-19-23, 12:59 PM
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People always talk about silicon bronze TIG brazing, but I'm not sure why you wouldn't be able to braze with other fillers. Heat control maybe?
If it's flowing out, I don't know why there would be a problem.
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Old 11-19-23, 04:26 PM
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Filling the gap with brass will improve cosmetics of it but I’m not sure if it will contribute greatly to strength, assuming the down is also sliced by a saw (as it seems). On the other hand, it is probably strong enough so filling in and filing would work.

Is your seat post merely scratched badly or is cracked, I can’t tell for sure!
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Old 11-20-23, 07:18 AM
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Originally Posted by Alan K
Filling the gap with brass will improve cosmetics of it but Iím not sure if it will contribute greatly to strength, assuming the down is also sliced by a saw (as it seems). On the other hand, it is probably strong enough so filling in and filing would work.

Is your seat post merely scratched badly or is cracked, I canít tell for sure!
thanks! I'm not sure what the seatpost looks like to be honest. This is for a customer who sent the photo via email. I'll need to inspect other parts of the frame.

I'm just a little cautious taking on repair jobs like this one. The consensus I'm reading is that, so long as the lug and ST are cleaned properly and the seatpost is long enough (short SP was possibly the original cause of the crack/sawing), then adding some brass will improve the aesthetics and the bike should be safe to ride (assuming there are no other issues). BTW - we think it's an old Cinelli.

Happy to see the discussion of tig brazing!
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Old 11-20-23, 08:41 AM
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The reason others have mentioned it looks like saw cuts is that cracks usually don't leave a gap, except under very unusual circumstances that don't seem to apply here. I suspect what happened is a seatpost got stuck, and it was taken out using the hacksaw method.
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Old 11-20-23, 08:57 AM
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I got some kind of bronze rod (not to be confused with what we regularly use like Gasflux C-04) from John Cherry - a framebuilder in Lafayette, Indiana (Purdue University). We were classmates in the 1st titanium tig welding class UBI had taught by Gary Helfrich in 1992. He had actually made ti frames before attending the class and I didn't want to be left behind if titanium was what everyone wanted. I used to paint John's custom road frames. This rod he gave me was supposedly formulated for tig brazing. With my Miller Maxstar 152, I could lay down a nearly perfect smooth bead. No little ridges (stack of dimes) and it was easy for me to get the same bead width all around the joint. I wasn't melting the base metal of the tubes, just the bronze (or whatever it was). The problem was that the joint had no strength. It took little to bust the practice tubes apart. I was never able to find out if another kind of rod would work that had greater strength. I would love to use tig brazing on our Ukrainian frames where we fillet braze the main joints. The tig brazing I did was faster with very nice looking results. I've done subject threads of this before without finding out much.
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Old 11-20-23, 11:27 AM
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People say that silicon bronze isn't very strong. I think that I have seen people say they use it for braze-ons though
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Old 11-20-23, 11:50 AM
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Originally Posted by unterhausen
People always talk about silicon bronze TIG brazing, but I'm not sure why you wouldn't be able to braze with other fillers. Heat control maybe?
If it's flowing out, I don't know why there would be a problem.
If you try and TIG with bronze rods designed for use with a gas torch they will fizz and pop and make a horrible mess. Maybe if you used flux as well it would be OK? Not sure. The people who design these rods know what they're doing. However if something does flow cleanly, as dsaul found, then it ought to be OK.
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Old 11-20-23, 11:52 AM
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Originally Posted by unterhausen
I have heard that tig and brazing don't mix. Two tig welders disagree?
Good point. There might be some brass rather close to those cracks. Could try TIG silicon bronze in an attempt to not get it as hot.
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Old 11-20-23, 11:59 AM
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Originally Posted by Doug Fattic
I got some kind of bronze rod (not to be confused with what we regularly use like Gasflux C-04) from John Cherry - a framebuilder in Lafayette, Indiana (Purdue University). We were classmates in the 1st titanium tig welding class UBI had taught by Gary Helfrich in 1992. He had actually made ti frames before attending the class and I didn't want to be left behind if titanium was what everyone wanted. I used to paint John's custom road frames. This rod he gave me was supposedly formulated for tig brazing. With my Miller Maxstar 152, I could lay down a nearly perfect smooth bead. No little ridges (stack of dimes) and it was easy for me to get the same bead width all around the joint. I wasn't melting the base metal of the tubes, just the bronze (or whatever it was). The problem was that the joint had no strength. It took little to bust the practice tubes apart. I was never able to find out if another kind of rod would work that had greater strength. I would love to use tig brazing on our Ukrainian frames where we fillet braze the main joints. The tig brazing I did was faster with very nice looking results. I've done subject threads of this before without finding out much.
For the union of tubes to be strong, a layer of the tube has to come very close to melting. If that’s not happening with the bronze rod (or whatever it was) you were using, I don’t see it withstanding occasional extreme forces the frame encounters. The only reason one gets away with brazing (flowing) using dissimilar materials is a large overlapping area.
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Old 11-20-23, 04:35 PM
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I'd braze it with nickel-silver, which is stronger than brass ('bronze") brazing filler, about the strength of some steels in fact. Slightly hotter melting than brass, but not a problem. I made one lugless frame, 531 tubes, completely brazed with nickel-silver in the '70s, and that frame was still being ridden last I heard, maybe 10-15 years ago (maybe still is). I don't actually recommend fillet-brazing with nickel-silver, but it has proven reliable if you want to. But it is excellent for repairs.

Semi-retired framebuilder and YouTuber Paul Brodie likes it, uses it a lot including for fillet-brazed frames, though he only makes a small braze with nickel-silver, and then he builds up the fillet with brass. Nickel-silver is difficult to file and smooth, more chance to accidentally thin the steel wall thickness, so brass is safer in that way, but nickel-silver allows you to make smaller fillets. If you don't file them at all, then that makes it a good choice. Of course for this repair, you won't be filing anywhere near the tube, only on the lug, so 100% safe.

For those who don't know, nickel-silver is a misnomer since it contains zero silver. I think the name comes from it being sorta silver-colored. It's actually a cousin of brass, containing mostly copper and zinc, but with a substantial amount of nickel which adds strength. You can use it with the same flux you use for brass, but I recommend the flux Cycle Designs makes specifically for nickel-silver. You can buy the rod from them too, or there are other makers. The stuff I used in the '70s was All-State #11. It comes as bare or flux-coated rod; I always use the bare rod, with paste flux. Flux-coated might be useful for a repair like this, but I haven't tried it.

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Old 11-20-23, 06:03 PM
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My simple fix would be to strip and clean it up. Then ream it, Then just do a little silver solder to fill any reaming cracks.

OH! ... And get a proper seat post too...
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Old 11-21-23, 03:01 AM
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Originally Posted by Doug Fattic
I got some kind of bronze rod (not to be confused with what we regularly use like Gasflux C-04) from John Cherry - a framebuilder in Lafayette, Indiana (Purdue University). We were classmates in the 1st titanium tig welding class UBI had taught by Gary Helfrich in 1992. He had actually made ti frames before attending the class and I didn't want to be left behind if titanium was what everyone wanted. I used to paint John's custom road frames. This rod he gave me was supposedly formulated for tig brazing. With my Miller Maxstar 152, I could lay down a nearly perfect smooth bead. No little ridges (stack of dimes) and it was easy for me to get the same bead width all around the joint. I wasn't melting the base metal of the tubes, just the bronze (or whatever it was). The problem was that the joint had no strength. It took little to bust the practice tubes apart. I was never able to find out if another kind of rod would work that had greater strength. I would love to use tig brazing on our Ukrainian frames where we fillet braze the main joints. The tig brazing I did was faster with very nice looking results. I've done subject threads of this before without finding out much.
I TIG everything and I use silicon bronze between where the two seat stays join at the top if it is too tight to weld it there. It's fairly easy to sort of fill that gap with TIG bronze if you finesse it a bit. I also use it for bridge tubes and to attach water bottle bosses, cable guides, seat-tube clamps, and all those things. I have made a frame (actually also making a second one now) with a "raw" finish and fake fillet brazing, where I did a normal TIG weld root pass, then silicon bronze (which does tend not to "dime-stack" like a weld, although you can make it do that to some extent) on top, which I then sanded. I doubt it needs the root pass.

In break tests I have done with it it's very nearly as strong as a TIG weld. If you join two chromoly tubes and then break them the tubes will be completely distorted and bent before the braze breaks. But usually the braze will break first before the tube itself (and this is not the case with a weld). I'm sure you could make a whole frame with it without any issues. On paper the UTS is very nearly as high as that of mild steel (I use a mild steel rod for TIG anyway) and the fillet is a bit larger to compensate for that. I think the reason nobody does this commercially is just that the rod is much more expensive. For an amateur builder I think it might work quite well as it is easier than welding-- much easier not to blow holes in the thin-wall tubing because you're using a bit less heat and putting lots of rod in there. You also might get superior fatigue life to a poor quality TIG weld which had defects in it. The rod I'm using is called "SIFSIL Copper 968". Off-topic but I also like it for car bodywork: less distortion on sheet metal and more forgiving when you're lying on the floor in an awkward position using a push-button instead of a pedal.
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