rustystrings61 

There is a lively discussion over in the C&V section regarding a mystery frame that appears at first blush to be a Swiss-built Allegro - the Bocama no.14 type II lugs, the distinctive wraparound seat stay top, the shape of the indents in the right chainstay, "diver's bell" chainstay cable stop, the sand-cast George Fischer BB shell and Swiss threading. However, this one appears to have IMPERIAL dimension seat and down tubes, as well as some other things that don't match, though they may well be changes made during a repair/repaint - no fender eyelets on the Campagnolo 1010 dropouts, likely added bottle bosses and top tube cable tunnels.

Is it possible to removed damaged METRIC 531 tubing, with it's 28.0 mm diameter, and swap in IMPERIAL 28.6? How about downtube replacement? Is there sufficient slop in the tolerances of the stamped steel Bocama lugs and the cast Fischer BB to permit that? I have been wondering if this bike started as an Allegro and required replacement of the seat and down tubes with whatever was on hand.

I'm pretty sure the Mike Melton-repaired Mercian I have involved subbing in metric Vitus 971 for the destroyed original Imperial gauge 531 top and down tubes on it, and I can see using smaller diameter tubes as replacements - but can it go the other way around?

unterhausen

Why did he replace imperial tubes with metric tubes?

I expect the bike you're asking about had the imperial tubes from the beginning. Even stamped lugs were relatively tight. I never used any Bocama lugs, but I doubt there was a difference.

I don't recall what lugs Trek used, those were nice. Other than those, the stamped lugs I have used include Prugnat, Nervex, and Cinelli. They usually have reamed ID.

bulgie 

The two systems are too far apart. Neither metric tubes in inch lugs, not the other way 'round, will result in clearances suitable to brazing.

The smaller socket can be milled or reamed to fit the bigger tube, as with putting a 1-1/8" tube into lugs or BB shell that were originally 28.0 mm*. But with a metric frame, there ain't no way to put in a 1" TT, since that's smaller than the existing 26 mm TT. No way to shim it and get a strong enough joint IMHO, the tube is just too small.

* I know at least one guy who did that. He ruined the seat tube on a beautiful Merz 753 frame with metric tubes. Unable to find a 28 mm replacement tube, he machined out the ST lug and BB socket to put in a 28.6 ST. Frame "good as new" except for the historical knowledge that was lost. Will future bike historians think Merz made frame with metric tubes except for the ST? Oh well, we don't need to worry about future historians when just trying to get a nice bike back on the road.

unterhausen

I want to put a dropper in a lugged bike. I haven't figured out how to do that yet without settling on a 27.2mm dropper. It doesn't help that all the dropper tubes are externally butted. Most investment cast lugs don't have enough wall thickness.

Kontact 

How about turning down the ends of a thick butted 28.6mm tube to fit 28mm lugs?

My Merlin's "butted" tubing is turned down in the center, and you can't see the transition.


Asking as a non-framebuilder.

rustystrings61 

Good to know. I will have to take a caliper to the frame and see what they actually are - the bike had been rebuilt and painted (badly) and had Vitus 971 stickers, hence my assumption that they were metric - but I'll double check now.

bulgie 

No go IMHO, the difference is too great, and tubing has too much "wow and flutter" to safely lathe-turn it down to such thin wall as you want on a lightweight frame.
Taking a 1-1/8" ST down to 28.0 mm with a target of 0.6 mm wall will require starting with 0.9 mm wall and removing 0.3 off the thickness. If you did it only at the ends to get it to fit the lugs you'd have a very heavy tube, not suitable for a quality lightweight. Turning it down over the full length would result in a plain-gauge tube at 0.6 mm, kinda weak at the BB end. Starting with a 1.2 mm / 0.9 mm single-butted tube might fix that problem, but I've never heard of such a thing.

The main problem is in the middle of the tube. The overall bend in the tube (which is never zero) will cause the wall thickness to be too thin on side and too thick on the other, possibly even breaking through to air on one side if the wow is enough. A follow-rest on the lathe can help with this, but it can't straighten a bent tube.

Centerless grinding is also a possibility, and it's probably what Merlin did. That at least partially takes care of the problem of bend in the tube. I don't have hands-on experience with centerless grinders, but I'm still very skeptical that you could take down the whole length of a tube that far in diameter, not as a practical matter. My assumption is, even if possible, it would be too expensive. And you'd still need to find that unobtainium 1.2/0.9 mm single-butted tube to start with.

Kudos for thinking outside the box though. It was a good question

Kontact 

It's removing .3 of wall, not .6. If you used 1.2 wall tubing you'd end up with .9.

And you could turn it with sandpaper, avoiding the straightness problem

And you don't need to grind the whole tube, just the ends.

bulgie 

Thought I was quick enough with the edit, to fix my arithmetic, but no such luck! I shoulda known the internet is too quick, no errors go unnoticed!

I have done a fair bit of thinning tubes by sanding, but I would never go that far. Just the time spent cleaning all the abrasive dust off the lathe afterward makes that job more time-consuming than it's worth.

And taking down only the ends is a nasty kluge IMO, not suitable for a quality lightweight. Any fame worth doing a frame repair on has to be a pretty nice bike to begin with, and I'd hate to end up with a reverse-butted tube, thin at the ends where it needs strength, heavy in the middle where it doesn't. Call me a snob, but I'd rather go looking for another frame that doesn't need a repair, before I'd do all that work to make this frame demonstrably worse. Sometimes ya just gotta walk away.

Kontact 

I was assuming 1.2/.6/1.2 tubing existed.
​​​​

bulgie 

Yeah that could be a useful tube, but never made that I know of. The largest butt differential I am aware of is 0.4 and even that is rare, most top out at a 0.3 differential (e.g. 10/7 or 9/6 or 8/5)

Kontact 

Companies like Fuji used to spec triple butted stuff like that to take brass brazing temps, and I assumed tig welders sometimes used thick butts as well.

But really, if the point isn't to make the frame just like factory but to replace a tube expediently (we are talking about substituting 28.6 for 28, after all), why not take a 10/7 and sand down the last inch that will go into the closed part of the lug. The pointed part can start to flare since the points can bend a little. So you end up with a butt profile that looks like 7/9/7/9/7 - but it will still ride like a 9/7/9 and shouldn't require an enormous amount of work - even if you sand it by hand.

If someone had the set up to braze it and liked the frame enough, I don't see why they wouldn't just do that. It isn't like the final product is going to look or ride weird if the sanding is decently blended.

guy153

You probably couldn't literally turn it down in a lathe for reasons discussed (tube too long, thin-walled, and not straight enough). Although maybe you could make an insert to go into the tube (to support it), grip that end in the chuck, support the far end somehow with the tailstock, and then cut off the bit you actually gripped-- you'd do all this at the end with the long butt. Don't know. It wouldn't fit in my minilathe anyway Could you sand it by hand? You could I guess but it would take a long time. 0.3mm doesn't sound like much but it's a lot for sandpaper.

bulgie 

Sounds like you have a different definition of "expediently" than I do. I still see it as too much work, to end up with a frame I would still see as worse than before, and inelegantly engineered.

For me anyway, it'd be easier to enlarge the lug and BB socket to 28.6 and slide in a factory-fresh SB seat tube. That results in a "good as new" frame, and I'm 90% sure it'd be faster (less work) than your way too.

That's assuming your way is even possible, which I'm not too sure about. To get your tube in, we'd have to spring the frame open enough to get the tube (with ends reduced in diameter) into both the BB socket and the seat lug at the same time, so that's springing the frame by a couple inches. I suspect most frames would yield under that much strain, so the frame won't spring back down to its original geometry after this process. Maybe it wouldn't yield, and you could get the new tube wrestled into place somehow (maybe with a ratchet strap going up to the ceiling?) but for me, the risk that it might yield, after I did all that work to prep the tube, just makes it a bad bet. Risky, and too little payoff.

As a courtesy, I gotta say, if you come up with some more speculation on how this could be done easily, I may not read it and probably won't respond. I think we've driven this one into the ground. Too hypothetical, too unlikely to result in any new useful ideas. You of course are welcome to keep trying, but I'm out.

unterhausen

I know ti builders grind the outside diameter of tubes in the center, but I feel like doing it at the ends of a steel tube is asking for cracks. Tbh, no bike is worth this. You can still get all kinds of tubes from BITD, you just have to religiously haunt ebay france and be willing to spend more than any bike is worth to get the tubes. You might be able to get someone to grind a 4130 tube, but see above about not being worth it.
There are builders out there that will do repairs and get paid pennies an hour, so I'm not saying nobody would do something like this. Sometimes I just shake my head at the repairs I see.

As a hobby builder, I look at repairs as a dead loss most of the time. I'd rather build what I want, and a repair is never it. Everyone I know that wants me to repair something has ridiculous ideas. But I don't do work for other people anyway. Maybe if I found an old Singer or Herse my size that needs work, who knows. I have always wanted an early '70s Colnago, but I'm not spending the money it costs to buy a good one and I don't think they are worth repairing. People always want too much for the busted ones. If, through some miracle of the internet, I did find a bike I thought was worth repairing, I wouldn't do any hackery, it would be as close to new as possible.

Kontact 

Oh, I thought one of the classic arguments for lugged bikes was the (relative) ease of replacing a damaged tube. I had assumed springing a tube in was just par for the course. If not, none of this thread makes any sense to me.

I really don't understand boring the lug out, unless you are planning to cut a downtube size hole into the headtube in the process.

unterhausen

I don't know if anyone ever built a frame with the idea that it would be easy to fix. I have definitely thought, "that would really suck to fix," when i was building one. But nobody is ever going to fix a frame because of something I did.

I think most builders are always moving on to the next frame.

Andrew R Stewart 

It usually takes me about half way through the current project frame for the next one to start really calling me. Andy

Kontact 

Apparently the way to do it is to remove the lower headlug, not to try and spring the tube in between two lug sockets:

https://www.velocipedesalon.com/foru...ent-36143.html

vintage cellar

So to me the important part of the original question was the belief that the OP had a Mercian somehow repaired with French tubing.
This is is a pretty ridiculous idea and could have been verified with a simple measurement.

Yes, Vitus tubing was made in France. But Vitus made goddamn Inch size AND Metric tubing.
The Metric crap was not that common in the U.S. BITD, and I'll bet most distributors tossed it in the dumpster by the mid 1980's.

It is almost impossible for me to believe that Mike Melton would have done such a Kludge-job. It makes NO sense - was there a shortage of inch-size tubes in the U.S. in the past 150 years that I somehow missed ?

Another Bike Forum classic posting. And how in the world amongst the 5000 or so Internet suggestions did everyone miss the
idea of Explosively Forming a 25.4 top tube to 26 mm diameter. How could we have collectively have missed THAT one ? LOL.

unterhausen

No framebuilder in this thread suggested anything out of the ordinary, and have said what you said with considerably more tact. Non-framebuilders stop in occasionally. If you regularly read this forum you probably would know who was who.

But your post did remind me that Bilenky has a swaging machine. I'm not sure if that could be used to make a tube smaller on the ends or not. Still wouldn't do it, but Bilenky might.