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I'm asking this question as I'm interested in the community opinion in the idea of "am I taking the right path" or "is there a better way".

tl;drl; - scroll to the bottom, please.

Namely, almost two years ago I've committed a (successful, in my opinion) conversion of cottered bottom bracket to a square-tapered one on a Motobecane 2040 steel road frame (late '70s or early '80s).

Why, one may ask. The reasons are:

  • I got the frame for a very attractive price and wanted to build a fixie on it (succeeded)
  • I hate cotters - they say you need only a hammer and a wrench to install and remove one, but in practice I never managed to do it properly (or I'm abusing my bikes). For square tapered cranks you need allen key or socket wrench for installation and a special crank remover for removal (sockets and allens I already had and the removal tool is like $5)
  • I was aiming for maintenance-free riding (also in bad weather), thus cartridge bearings were a very good option. If the set gets excessive play I'd rather replace the bearings or the complete cartridge than overhaul the ball bearings, set the play, check it and adjust after some time, etc.

So what was the challenge?

  • it is a Motobecane frame, where the BB threading is neither typical French, nor BSC (a.k.a. BSA), nor Campa, hence an off-the-shelve cartridge bottom bracket set with cups (either nylon or aluminium) is nearly unavailable
  • there are BSA axles available, but the BB shell on Motobecane is a bit wider so without replacing the balls it won't work. And again - it wouldn't be weatherproof.
  • the drive side cup was seized (i didn't have impact wrench then)

After an evening with Vernier caliper and internet, sourcing for affordable parts I've came up with a brilliant idea - I would keep the original cups and find a BB cartridge that would fit them. It was to be done. the cross-section of the whole concept (proven, working, nota bene) looks like this:
Mike BB conversion
Black is the BB shell,
Red are the original cups (with bearing races),
Green is the Miche square-taper bottom bracket cartridge.

Positioning of the axle is done by placing some shims (ground to size) on left or right side.

The whole setup is working correctly now for more than 1000 km. The retaining nut came lose once, in fact I've assembled everything to check my proof-of-concept and it was working so well that I forgot to tighten or thread-glue the setup. After fixing that I'm experiencing no problems whatsoever.

Soon I'm going to build a daily commuter for my daughter based on Raliegh Ladies Traveller (something between the '30s and the '70s) frame (which we got for free and she really liked shape of the frame and front fork). As it is Raleigh, the bottom bracket has proprietary threading, which isn't BSC (a.k.a. BSA) - I've already checked it.

TL;DR; reference: My question is:

  • shall I go the same way I went for my Motobecane, or
  • do you know some better way to convert the bottom bracket for the frame with non-standard cups?

Btw. both cups (undamaged) are now degreasing in a bucket, threading in the BB shell is also in very good condition.

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Depends on

  • How much you value originality,
  • The thickness of the metal at the cup threads
  • The internal diameter of the cup hole.

Either keep it totally original, which means scouring ebay for old bike parts (expensive)

OR look at retapping the BB shell to accept a modern cartridge bearing unit. This is probably beyond the home mechanic because it will require a large tap and a way to keep it completely straight. A frame builder might be your best contact. A BB cartridge is cheap, common, and easy to change IF the threads match, and the width is workable.

OR see if there's some combination of press-fit BB that will fit in the BB shell. I'm not familiar with these, but there was one where two parts entered and threadded themselves together in the middle, allowing a hollowtech axle to go though. Would increase Q factor and might look a bit weird.

OR your proposed hack should work just as well on the Raleigh as on your bike. Downside is if it needs servicing and you're not available to do it.

This is an interesting question - please consider posting your own answer, ideally with progress pictures.

  • 1
    I'm not aiming at originality, this bike is supposed to be a daily ride, I value reliability more than originality in this case. Retapping is also an expensive option, furthermore there may be not enough "meat" in the BB shell for the standard BSC threading. I'll have a look at press-fits. – Mike Apr 25 '18 at 7:23
  • @Mike for long-term convenience, you want to end up with a boring standard 68mm english threadded BB, so that commonally available cartridges will fit easily. – Criggie Apr 26 '18 at 2:53
  • you're correct. I've measured the BB shell - it's 77 mm wide, so enough room to file it down to 68 mm. What about re-threading it? Do you have any experience with that? I'm afraid if I run the standard BSC tap on it it would be like cross-threading it. – Mike Apr 26 '18 at 9:29
  • I have no personal experience with this. At this point you might be best checking with a framebuilder. – Criggie Apr 26 '18 at 10:16
  • Thanks for the suggestion. Given the difficulty in finding a reliable framebuilder within reasonable distance and their rates I'd rather become a framebulder myself. Furthermore, with the surplus of the bicycles where I live, the advice given most often is "throw this frame away and find an another". – Mike Apr 26 '18 at 10:39
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I've came up with two possible solutions. They are both theoretical and I'm not very fond of them. But unless anything else comes up, I will try one of them.

Solution 1 is to find a bottom bracket with nylon threadless cups and press-fit them into the frame.

Pros:

  • "bolt-on" solution
  • does not require specialised (i.e. expensive) tools

Cons:

  • when it comes up to servicing the bottom bracket, the cups will most likely be destroyed, hence each overhaul, not often though, requires new cups
  • the shell probably needs to be narrowed to accommodate standard cartridge spindle (thus reamer required, read - expensive)

Solution 2 is to re-thread the frame (after narrowing the shell to 70mm) with Italian threads, as those are 1 mm more in diameter than Raleigh.

Pros:

  • still very serviceable (cups are easily removable)
  • bottom bracket spindles Widely available

Cons:

  • requires reamer and tap (or framebuilder visit, read - expensive)
  • not sure whether there will be enough "meat" for new thread in the shell with just 1 mm diameter difference.

Again, these are theoretical solutions and still finding the correct spindle to go with my Motobecane solution seems the most reasonable.

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