I've posted this bike to this forum before, but I need some more detailed information on the specifications than last time. The bike is a 62 cm Motobecane Super Sprint made with 2040 tubing. Diacomp breaks, Aero Coronado cranks, Thuret derailleurs and shifters.

I've been scouring the internet for details on BB threading and spacing, but can't find it anywhere. I've read in several places that older Moto's such as this one had Swiss threading. If so, does this mean that I will need to get a new BB before I can install updated cranks? Will any BB work as far as a replacement is concerned?

Any help will be much appreciated.

There's a good chance it's Swiss-threaded.

First of all, why change the crank?

Secondly, if you have to change the crank, you may not have to change the bottom bracket. So don't worry.

Thirdly, if you have to change the bottom bracket, there are threadless designs so you don't have to find a Swiss-threaded one.

Tom

Bottom bracket cups are generally stamped with the thread standard. Are there any markings on them?

I'm changing the cranks because I'm converting to SS (I'm not sure how dirty of a word 'converting' is on this forum). I would just get a new chain ring, as I like the cranks that are currently on the bike, but the 52 tooth ring is a part of the spider (the 52 tooth is inseparable from the spider). SO, the issue becomes, if I want to replace the cranks will I have to replace the BB because of the odd threading? Will I be able to find cranks that are compatible with the Swiss threading?

@ T-Mar: I don't have the tools to get the cranks off properly to look at the BB. If I can't get any help here I'm going to take it into my LBS and have them help me. The LBS's in my area are just so expensive even when it comes to minor things; they aren't really interested in helping anyone out unless you're a roadie.

More answers lead to more questions.

Why convert to SS? It's a fad I don't understand. SS is light and simple, but not drastically. Your bike isn't terribly heavy, nor is it that unreliable. The amount of trouble to convert is greater than the amount to maintain it, for at least a good amount of time, so the simplicity argument doesn't hold much water.

Cranks are cranks, and they don't know what threading the BB is. The tricky part is to get the right crank the right distance from the frame so that the chainline is right. And there are few reliable formulas for that. In fact, it's one of the trickiest things of bike maintenance. It's possible that replacing the next cranks you try will be a drop-in replacement and your BB can keep on spinning. And even if not, it's possible that you'll replace the spindle and leave the cups in place. So Swiss threading is really a small obstacle compare with setting the chainline. And given that setting the chainline can be so tricky, I have to ask again why convert.

Sorry I'm questioning so much, but the basic assumptions have to be explained before the small details can be.

The thread standard dimensions could be on the cups around the spindle. A magnifying glass might help to identify it even if the crank is still on.

I'm converting for simplicity's sake, and I think the arguments do hold ground. First is the issue of weight. To me, the bike is incredibly heavy. I know a lot of that comes from the less than stellar tubing, but I know dropping the derailleurs and gears will help. Second is efficiency. The drive chain on this bike is not performing all that well. I'm still struggling with the friction shifters, both front and back. Third is maintenance. I'm much more amenable to maintaining a simpler machine like a SS than I am a geared bike.

Yeah, I knew that getting a good chainline would be the most difficult task. The wheels I have for the conversion are 120 mm spacing. As far as fit with the BB is concerned, though, any crank will work? Is the BB definitely square taper?

EDIT: I couldn't see anything on the cups. Here are some pictures.