Bicycle Mechanics - Conversion of bottom bracket thread
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08-22-11, 01:33 PM
so i got this old steel frame - Motobecane - that i would like to build up. comes with fork, too.
nowadays it is kinda hard to find the original FRA threaded M35x1 mm bottom brackets. if one looks at the measurements, they are pretty similar to the ITA standard. the plan is to have the buiild take camapgnolo chorus ITA cups.
has anyone ever done a conversion? i'm aware the ITA uses a 70mm wide bottom bracket whereas the FRA is 68mm ... maybe a spacer (from XTR) on each side?
curious about any comments on this...
08-22-11, 02:31 PM
You can buy JIS French thread bottom brackets brand new from velo-orange (http://store.velo-orange.com/index.php/components/bottom-brackets/grand-cru-bottom-brackets-hollow-axle-alloy-cups-26.html).
From your description I'm not sure if you were planning to go with a modern 102mm cartridge or just older loose ball cups. If it's just cups you want, I've found Sugino french cups on ebay for very reasonable prices (<$20).
But if you really want modern Campy, you can have the BB rethreaded Italian. You can't just force the cups in. Add a 2mm spacer (mine's on the drive side on my conversion) and you're good to go.
08-22-11, 03:58 PM
Is there a problem with the original cups that you can't continue to use them?
As noted above, French thread cartridge bottom brackets are available. But be aware that some Motobecanes from the late 70s and early 80s are actually even more obscure Swiss thread. Swiss thread is 35mm x 1mm like French, but the fixed cup is left hand thread. AFAIK, there are no modern sources for Swiss thread bottom brackets. If you do find your frame to have Swiss thread, Velo-Orange and some other companies offer threadless bottom bracket cartridges. Personally, I would consider this solution before reaming and tapping your frame to Italian thread.
And BTW, your fork is also likely to be metric 25mm x 1mm thread rather than ISO 1" x 24tpi. If your present headset is not usable, Velo-Orange offers a metric headset as well. Your other option would be to replace the fork with an ISO thread fork to use an ISO headset.
08-24-11, 01:19 AM
Thanks a lot for the tips guys! My idea was not to use the old-style square taper axle but the newer system. So I'm having the LBS re-thread the bb to ITA and use the spacers. Even though I live in Switzerland, the swiss thread (rh/lh) are a rare find and I don't want to tear apart the old Cilo ;)
I wasn't aware of the fork thread issue but luckily the headset is pristine and I can re-use :)
08-24-11, 01:28 AM
nice website btw :)
08-24-11, 02:06 AM
In re-threading to Italian you have to ensure that the wall thickness of the bottom bracket will allow this and with velo orange offering French threaded bottom brackets and headsets it seems like re-threading is an option that one would use if they had nothing else left as an option.
This may have been true some years ago but even then, French threaded bottom brackets and compatible spindles were available to anyone that might have looked or them and now velo orange makes things easy.
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