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Lug fitment, how much gap is too much?

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Lug fitment, how much gap is too much?

Old 11-24-14, 10:13 AM
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bill meyer
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Lug fitment, how much gap is too much?

I'm building a frame for my father and have a old set of MTB style lugs that you can't find anymore, think I got them from Nova years ago, but they're perfect for the cruiser/city bike we're building up. Anyhow, I dusted them off, ordered some tubes and, and have brazed up the front triangle. Now I'm second guessing the entire thing as some of the castings were poor. For instance, on the BB shell, the DT lug was slightly oval and had gaps of around .3-.5mm on the sides and I had to use some 45 silver to fill those. The rest of the shell was fine and 56 flowed through nicely. Same issue for the DT-HT lug, the HT band was slightly large creating about a .3mm gap all around. 56 actually filled it fine, but I've read that at those thicknesses the silver can crack over time. Any experience with this? Should I scrap it all and start over?
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Old 11-24-14, 11:03 AM
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rellis
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That is a large gap for silver.

Check out Cycle Design Silver Brazing Wire Cycle Design

The fillet pro rod will fill that. Just brazed in some stainless dropouts with it. Filled the stay like brass (costs a lot more!).
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Old 11-24-14, 11:06 AM
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unterhausen
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fillet pro is great stuff, but 45% silver is a suitable substitute. I would just keep an eye on the frame if you're confident in your brazing. It would have been better to close the gaps by a little blacksmithing before brazing
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Old 11-24-14, 11:15 AM
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bill meyer
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I did close up the gaps as much as possible, but the BB shell casting on the sides was just too stiff for me to do much with. The HT bands are simply too large, closing up one area just ovalizes it somewhere else. Can you work\blacksmith that smaller? I'm confident in full silver penetration, just a little worried about how thick it is in places.
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Old 11-24-14, 11:32 AM
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Live Wire 
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Could've made a cut in the headtube bands and squeezed them down that way.
you probably used a headtube that was thinner than what the lugs were made for.
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Old 11-24-14, 01:59 PM
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bill meyer
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Originally Posted by Live Wire View Post
Could've made a cut in the headtube bands and squeezed them down that way.
you probably used a headtube that was thinner than what the lugs were made for.
I did consider that, but then I was worried about the integrity of the band and it possibly splitting at the cut... who knows though.
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Old 11-24-14, 02:27 PM
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I have cut and welded stamped lugs. Not sure how well that would work on a cast lug, but then again you probably don't really need to make a welded seam. If you put the seam out on the front of the lug somewhere, it's not going to fail at that point assuming the brazing is good
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Old 11-24-14, 02:49 PM
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Was brass considered when the gaps were seemingly too large for silver? Andy.
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Old 11-24-14, 08:02 PM
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bill meyer
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Originally Posted by Andrew R Stewart View Post
Was brass considered when the gaps were seemingly too large for silver? Andy.
Yes, but I've never worked with Brass and figured if the silver flowed and filled the gaps then it would be ok, now second guessing that of course. It's only frame 2, so lots to learn still.
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Old 11-24-14, 11:31 PM
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With any lug there will be both high and low spots of gap. Some of the skill is to even these out as best you can. Between black smithy and grinding one can get pretty good fit up if patient. But as one goes through the prep it can be clear which filler will be used.

One test I learned a long time ago was how a lug will slip down a vertically held tube. If the lug only slips easily but for a short bit down a tube after the tube's end is tapped on a bench top then the fit is likely a silver one. If the lug slips down the tube a lot after tapping or slips without any tap then brass is in order. Andy.
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Old 11-25-14, 04:12 PM
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I like that test Andy, thanks. This particular BB required tons of cleanup to even get the tubes inserted. I removed too much material in some areas, not enough in others, hard to tell exactly where it's binding sometimes. Cheapo lugs are not kind to beginners..

I have a set of Pacenti lugs for #3 and those are nearly spot on right out of the box, so I feel like I'll be miles ahead on the next build.
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Old 12-06-14, 04:44 AM
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OP; A long time ago I snapped off a 1" HSS drill bit at work (for rail road). I tossed the broken bits into the tool box. Years later, the butt end of that bit ended up in the corner of the room with the frame tools. Clamped down tight in the vise, it has been really useful as an internal anvil for beating out wonky lugs to get them right enough to use. You might want to consider digging around for a piece of steel about that diameter for future projects...

/k
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