French Threaded Shifter Bosses

While I usually love all things French, I was perplexed to discover that the shifter bosses on the early 1980’s Méral Randonneuse I am currently restoring are not threaded “normally”.  What is normal threading for a shifter boss?  Well, it’s the same  5 mm x .8 that you will find on much of the rest of your frame:  bottle cage mounts, rack mounts, etc.

Courtesy of Park Tools.

Fasteners have several elements that help identify their size, the two most important of which are: thread pitch, which is the distance from the crest to the crest, and thread diameter which is the outer measurement of the thread crests.  English threading is designated by the frequency of how many threads are counted along one inch.  This is know as “threads per inch” or TPI.  Metric threading uses the direct pitch measurement in millimeters, measuring between two adjacent thread crests.  A fastener designated as TPI is “standard”.  Unfortunately, some fasteners are “mix and match”, with both a TPI and a metric size, such as Italian bottom bracket threads labeled “36mm x 24 tpi”.  There are standard coarse metric threads which are designated with the letter “M” followed by the thread pitch.  For example M6 = 6.0 diameter with a 1.00 mm pitch.

This Méral has a shifter boss on the seat tube – for engaging a bottom bracket mounted dynamo.  When I acquired the bike, the lights and dynamo had been removed from the frame.  So, I planned on reinstalling a BB dynamo hooked up to a friction shifter on the seat tube.  That’s when I discovered that the threads on the shifter mounts did not match any threads on any shifters I tried, even including some Simplex shifters from this era.

Simplex Shifter with 6 mm x .8 threading

That’s because these shifter bosses are tapped with 6 x .8mm threads (not 5 x .8), as you can see from the above nomenclature on the bike’s Simplex shifter boss bolts.  These Simplex shifters use a nonstandard threading on the boss. My research indicates that Simplex used a variety of threading standards for its shifters:  5 x .8, 5 x 1.0, as well as 6 x .8, shown above.  Classic Rendezvous has a discussion of these sizing anomalies.

Soubitez BB dynamo

Here is one idea for a BB dynamo for this bike – a Soubitez which is in good shape. I’ll need to set up the seat tube shifter, and given that the boss threading is non standard, it’s back to the drawing board for now.

6 thoughts on “French Threaded Shifter Bosses

    • Daryl – I would consider if I trusted myself not to mess up. There’s very little material on the bosses, so one mistake could be very expensive. Instead, I will try to source some bolts with this threading and use them with an existing Simplex set I have, which has standard threaded bolts.

      • Thanks for the reply. Thread sizing is the bane of the bicycle restorationist. Good luck. Maybe a machinist can help you out. All the best, Daryl

      • Yes, a machinist could help by making the 6 x .8 bolts. Or, I may try tapping only the seat tube boss (since I already have the proper bolts for the downtube bosses) to a 1.0 pitch. That would mean using a larger, but standard M6 bolt, which might work with standard shifters.

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