Originally Posted by
dweenk
If it is 1971, it may be French threaded. Do you know the make of the crank? Then you need to determine if the taper is ISO or JIS.
dweenk, not a personal assault...
The ISO - JIS issue is pure bunk... Part of cycling urban lore that's been passed down since the early 70's. BITD Phil Wood BBs came in one style taper for all.
On brand new crank arm you can run into some problems mixing and matching but once they've been mounted it doesn't make much difference (see first picture in my Flickr link below).
Reason, aluminum is soft and malleable. Once the crank has been installed on a square taper spindle the metal deforms. Every time it's removed and reinstalled the distortion gets more profound.
Another consideration, manufacturing tolerances on both the spindles and the square holes in the crank arms varied widely even on top quality brands. Until the 1980's most European bicycle components were produced using equipment that survived WWII - some even dating back to before WWI....
I threw this little Flickr album together about 4 years ago to document my efforts to build a single speed using an old Gitane frame.
Just for yuks, I measured about 35 spindles that I had plus at least 10 sealed bearing BBs.
Bottom line, I it fit's it works.
Please read the descriptions under the pictures for explanations.
https://www.flickr.com/photos/282672...album-72157627
Until the late 1970's (maybe the early 80's) most Motobecanes used me metric threaded BBs. A few had Swiss metric LH threads on the fixed cup - no way of telling unless the cups are marked (Campy, some Stronglight and Japanese makes).
Originally Posted by
dweenk
I can't tell from the photo if the chain rings are worn, but given the age of the bike-that is likely.
If the bottom bracket is OK, then you would only need to replace the crank arms and chain rings. It appears that the drive side crank arm is swagged to the large ring, so they cannot be separated. If you live in a large metro area a bike coop may be you best bet. Make sure that your problem is not the chain before dumping time and $$ into this.
+1
The crank looks like a lower end Nervar. They came on a lot of the less expensive French bikes in the 1980s (Moto, Gitane, Peugeot).
[MENTION=333224]juvela[/MENTION]
Could be a worn out chain and or freewheel. That will cause those problems too.
verktyg
Chas.