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Old 09-23-07, 09:38 AM   #1
anomaly
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What BB for older motobecane?

I have a friend who stripped a Tourny crank arm on his Motobecane, and has tasked me with replacing the crank arm (which has proved difficult) or the whole crankset. Would it have a 68mm shell, if so what threading? Or if I kept the bottom bracket what taper crank would I need?
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Old 09-23-07, 02:01 PM   #2
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What vintage? If 80's it is probably BSC, and probably marked to indicate somewhere on the frame. The Japanese crank is also a strong indicator for BSC. It would almost surely be a 68. If it is newer, then the width can wander. If its an old one, it may be French, as the Japanese built to both standards. Dunno when Tourney was popular.
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Old 09-23-07, 06:31 PM   #3
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Moto changed from French to Swiss BB threading sometime in the mid-1970s, before Peugeot did, but I believe both companies went to ISO by the mid 1980s. The square crank taper itself is always 2 degrees, but Shimano cut its cranks to a fatter end width than everyone else. Circa 1994 Campagnolo reportedly copied Shimano.
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Old 09-23-07, 10:04 PM   #4
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ISO is BSC, I think...the vast majority of the Japanese parts in the world are ISO, but they supported most of the standards to some degree.
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Old 09-23-07, 11:59 PM   #5
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Originally Posted by luker View Post
ISO is BSC, I think...the vast majority of the Japanese parts in the world are ISO, but they supported most of the standards to some degree.
Can somebody explain the alphabet soup?
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Old 09-24-07, 07:09 AM   #6
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Can somebody explain the alphabet soup?
Who better than Sheldon?

http://www.sheldonbrown.com/gloss_ta-o.html
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Old 09-24-07, 09:37 AM   #7
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Be aware that there are a couple of places in that article on threading that are misleading. If you go based strictly on the data in the charts, you'll be OK, but the written text that can cause some confusion if you try and relate it to the charts.

There are three common thread systems, not two as Sheldon states. In addition to metric and SAE/American (i.e. uses English units of measurment), there is a hybrid system that combines both English and metric units of measurments. For instance, Italian bottom bracket and freewheel threads are 36mm x 24 TPI. That's a metric nonimal diameter combined with a English thread pitch measurement.

He also states one of the systems as being "metric (ISO)". ISO is not exclusively metric. When the International Standards Organization was chosing the bicycle threads standards, they looked at the technical merits while considering the financial impact. In other words, they wanted the best thread, but not if the majority of the bicycle manufacturers would have to change. Consequently, ISO selected standards from both English and metric systems, depending on the application. For instance, the ISO bottom bracket thread standard, uses an English measurement system, while the ISO axle thread standard is metric based.

Note that an ISO English based system is not the same as English threading. While they are compatible, there are minor differences in the thread profile and sometimes the nominal diameter.

Last edited by T-Mar; 09-24-07 at 09:44 AM.
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Old 09-24-07, 03:58 PM   #8
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From: http://www.sheldonbrown.com/gloss_ta-o.html#threading :
Threading Systems
For normal, generic nuts and bolts, there are two threading systems in common use, S.A.E. (U.S.Standard) and Metric (I.S.O.). S.A.E. threads are designated by a diameter and a number of threads per inch (TPI). For example, the common 1/4 - 20 means a bolt 1/4 inch in diameter, with 20 threads per inch. Metric bolts are sized by the diameter and the distance between adjacent threads, for example 6 x 1 refers to a bolt 6 mm in diameter with threads 1mm apart. (This is the size used for brake mounting bolts and brake shoe hardware.) Similarly, 5 x .8 means a bolt 5 mm in diameter, with threads .8 mm apart. (This size is used for fender and bottle-cage fittings, shift lever bosses, and most cable anchor bolts.) A third system, known as Whitworth, was used in Britain up until the 1960's when they converted to metric.

Bicycle parts come in even more different thread systems than common nuts and bolts. There are different standards for headsets and bottom brackets for American/BMX/OPC, British, French, Italian, Raleigh and Swiss bicycles.
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Be aware that there are a couple of places in that article on threading that are misleading. If you go based strictly on the data in the charts, you'll be OK, but the written text that can cause some confusion if you try and relate it to the charts.

There are three common thread systems, not two as Sheldon states. In addition to metric and SAE/American (i.e. uses English units of measurment), there is a hybrid system that combines both English and metric units of measurments. For instance, Italian bottom bracket and freewheel threads are 36mm x 24 TPI. That's a metric nonimal diameter combined with a English thread pitch measurement.
The first of the two paragraphs above is referring to "Normal generic nuts and bolts" The Italian thread system is a non-issue for nuts and bolts.

The second paragraph, beginning "Bicycle parts come in even more different thread systems than common nuts and bolts" does mention the Italian (and other oddball) threading as used specifically on bicycle parts, such as bottom brackets, headsets, hubs.

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Originally Posted by T-Mar View Post
He also states one of the systems as being "metric (ISO)". ISO is not exclusively metric.
For "Normal generic nuts and bolts" ISO is exclusively metric. That's what that paragraph was talking about.

Quote:
Originally Posted by T-Mar View Post
When the International Standards Organization was chosing the bicycle threads standards, they looked at the technical merits while considering the financial impact. In other words, they wanted the best thread, but not if the majority of the bicycle manufacturers would have to change. Consequently, ISO selected standards from both English and metric systems, depending on the application. For instance, the ISO bottom bracket thread standard, uses an English measurement system, while the ISO axle thread standard is metric based.

Note that an ISO English based system is not the same as English threading. While they are compatible, there are minor differences in the thread profile and sometimes the nominal diameter.
That's correct.

I have "bolded" the sections of each of those two paragraphs that differentiate what they are addressing. I hope this helps clarify the respective issues discussed in those two paragraphs. If not, and you have any suggestions for improved wording, I'm glad to consider them.

Sheldon "Revision Is Easy On The Web" Brown
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Old 10-22-07, 02:05 PM   #9
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My Moto from the mid-70's has a 68 mm Swiss-threaded BB
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