Motobecane bottom bracket confusion...
 

There is some discussion on this in a sale thread but figure it could use its own thread rather than be lost in the sales forum.

Building up a 1976 Motobecane Super Mirage mixte for someone. I've got a frame and no BB. I was told here that if it's not french threaded it is likely Swiss. Either way, not English. At a LBS they checked it out and told me that an english threaded BB would work. I was very skeptical and told them its likely french or Swiss. They didnt know what Swiss threading was. They tested it and said it was all good. This picture if the result. Drive side went in just fine counter clockwise. NDS went in fine clockwise up to a point. This is how much threading is left sticking out. I know a little is bound to show, but this seems like a whole lot to me. They told me it was normal.

The crankset is Stronglight 93, on which I will only be using the 45t ring. This BB is 68x103.

As this bike is for someone else I want to make sure it's all done correctly and safely. Thoughts?

https://cimg9.ibsrv.net/gimg/bikefor...2dfeb6236a.jpg
https://cimg0.ibsrv.net/gimg/bikefor...bb2b781794.jpg

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if you are confident of the date then it could not be BSC unless someone has altered it

people at bike shop sound uninformed

BSC cups will start to go in to a CH shell because their pitch diameter is 34.85 and metric/CH thread is 35.0mm pitch diameter

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Is there a chance damage has been done to the threading at this point? And I'm not 100% on the date but at least 80%. It's between 1975 and 1977 for sure.

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those dates are definitely too early for BSC shell for MB

damage to shell:

one thing which could always be done is to run a set of CH taps through to clear/reform threads and then fit a CH bottom bracket assembly


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I swear this is the last french single speed conversion I take on for anyone :troll:

Looking to pick up this which nlerner used in a similar era Moto, but unsure if 110 is an ok size or single speed with the Stronglight crank: https://www.amazon.com/Sunlite-Threa...NXM?th=1&psc=1

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the OEM Stronglight spindle used with the model 93 chainset would have been either model 118 or 120 for a two plateau drive train

since you are going single plateau one would expect an ideal chainline to be achieved with a spindle several mm shorter than the 118-120 size

the design of the cartridge you linked to appears not to permit one to fine tune their chainline as is possible with a system such as the Phil Wood


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Which may also make the 103mm that the LBS gave me far too short. I just threw an arm on and it's mighty close to the chain stay in rotation...

Would you recommend 116 or 110?

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Me "guesstimate" would be that the 110 would be the better size.

One jumbo caveat: have no experience converting two plateau drive trains to single plateau. ;)


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In that other thread, you mentioned using a mod. 93 crankset with a single ring. Those need a fair amount of clearance because of the “shoulders” that hold the inner ring. I’d probably opt for the 116 though maybe you can get away with the 110 if you use a couple of 1mm spacers on the drive side.

Stronglight used a 113 mm spindle for their track bottom bracket setups - I ran one for years with a mod. 93 that I shaved down the inner ledges - the Trek I was using it with was tight on clearances. I ran the chainring in the outside position with good chainline that way.

i am currently running a Phil Wood unit I snagged used with French rings - but I would be better served with a 107 Phil.

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that reminds me -

the T.A. spindle for single plateau is Nr. 314

the Phil Nr. 1 has an overall length of 108

and the Nr. 2 and overall length of 113

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Does “threw it on” Include torqueing? If it was properly torqued and not hitting the stay, I wouldn’t go much longer for the spindle.

Swiss threading is very likely on a Motobecane of that vintage. They stayed with hard metric threading, but finally got the fixed cup thread direction right (actually left). Peugeot belatedly caught up a few years later -- I was surprised to discover that my 1980 PKN-10 is Swiss-threaded, rather than French.

This is where it's at. So if this is 103, I'm thinking the 106 above will be ok. The 110 will likely be too much.

https://cimg3.ibsrv.net/gimg/bikefor...601e2d6e05.jpg

Welp, time is very much of the essence so I’m ordering the pricey one from VO as the 106 on amazon has sold out and the shipping time is several weeks anyways

The last time I did a similar build (Peugeot mixte early 70s with the same single ring Stronglight 93) it looks like I used a 113 (just went back and looked at that order), which is surprising...but it worked.

So based on the photo above which is 103, should I go with that or 110?