Columbus Head Tube

Hi everyone!

I've got a Columbus tapered head-tube (code-name CYRT18250) with braze-on cups (ZCALT46UP, ZCALT56DOWN) for a mountain bike project, and have a couple of questions.

First the headtube instead of being nice and shiny is covered in some black stuff. Anyone know what that is or the preferred method of cleaning it off?

Second, after cutting it to the right length should I face it and solder the cups in before building the frame, or is it better to do that afterwards? I'm thinking before because if anything it will reduce distortion from welding. If things do distort much it's not going to be perfectly round either way and it's just going to make it harder to get the cups in if I do it afterwards.

Any tips for the best way to solder the cups in also appreciated :) I don't think strength is very critical, it's just to stop them moving around the same as a normal press-fit would.

Welcome to V Place.

Can we see a first and last name with each post on Frame Forum?

Thank you.

I would add the cups firs because they are, in essence, reference points for alignment. If you lack experience and want to cover the most bases in the shorter amount of time, add them before the frame is assembled.

If there are welders reading and have another POV, please chime in.

Thanks for the response. After practising on some test tubes I have decided to braze the cups on afterwards. Since the lower cup goes quite far into the tube there's a concern that the braze might be close enough to the subsequent weld of the downtube to melt if I weld after brazing. Also there's a lot of flux residue and junk to clean up after the brazing. That's going to do need doing anyway before painting but I wanted things as clean as possible for welding without any risk of anything nasty or sparky around there.

The main triangle is all welded out now and looking good but I think I'll leave the cups until after the rear triangle since it fits in my jig better without them.

I managed to cut the HT pretty square with a hacksaw and some filing and took it to a couple of places to try to get it turned on a lathe, but they both said the tube was too thin to clamp it properly in the chuck. I guess you'd need to machine bit of bar to fit in the middle first. Anyway the second place did a pretty good job with a belt sander and checked it with a height gauge on a flat surface so I think that should be good enough. None of the facing tools I have are big enough for either end!

In terms of alignment, what I need for the bearings to work properly is parallel to somewhere less than half a mm, which is somewhat inside the tolerances of my home-made and rather redneck jig anyway.

It occurs to me that the black coating on the HT might even be something you're supposed to leave on for the brazing that makes clean-up easier. But without knowing that for sure it's safer to just clean the whole thing up properly and get it shiny and bright which is what I did. I'll experiment some more with the off-cut :)

Guy Gadboit

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Originally Posted by guy153

It occurs to me that the black coating on the HT might even be something you're supposed to leave on for the brazing that makes clean-up easier.

Guy Gadboit

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No.

Super no.


- Garro.

Good thing I cleaned it all off then :) Still curious about what that stuff is though. Well I finished the frame and put the cups in last. Some way off the perfect brazing job mainly because I suck but they only really need to be tacked in there, it's not like it's a fork crown. The lower headset cup was a bit of a tight fit (and loose in other places) due to some ovalization of the fat end of the HT during welding, which is not surprising given how fat it is. Probably too tight for the braze metal to flow in in the tight spots and too loose for it to stick properly in the loose spots, but I don't have that excuse for the upper cup which still fit up well. I used plenty of flux and got it all clean and dry afterwards fairly easily.

So I don't know maybe next time I'll do the cups before welding, but I don't know what issues I would have found that way around, and good welds on the HT is still always going to be the top priority.

Guy Gadboit

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Originally Posted by guy153

Good thing I cleaned it all off then :) Still curious about what that stuff is though.



Guy Gadboit

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It's burned / melted on oil and such from manufacturing

Generally referred to as "Mill Scale"

- Garro.

I think you're probably right it's some kind of oil residue. But that's not mill scale. Mill scale is iron oxides and is found on "hot-rolled" (or "black" as it's misleadingly called in the UK since it's blue or grey) steel. It needs at least a flap disk to get it off. But unless you were Walmart you wouldn't be making a bike out of that stuff. All quality bicycle tubes are extensively cold-worked and any mill scale from the manufacturing process is a distant memory, probably washed off with acid before they even started drawing and butting and so on.

The stuff on the tapered head tube came off easily with a bird's nest wheel. I think there's something similar inside the other tubes. So maybe it's from whatever kind of lube they use in their drawing/butting machines. They polish it off most of the tubes afterwards, which is why they're nice and shiny, but for some reason they leave it on the outside of those tapered HTs. Maybe it just doesn't fit in their polishing machine!

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Originally Posted by guy153

Maybe it just doesn't fit in their polishing machine!

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Bingo.

Technically you're right about mill-scale but everyone I know calls what you're describing as mill scale. Shitty black crap on tubes = mill-scale. It needs to come off before welding/brazing. You'll find a lot of tapered, short or bent tubes won't have it polished off.