French steerers are 25mm with 1mm pitch threads. British/ISO/Japanese steerers are 25.4mm diameter with 24 Threads Per Inch pitch threads.
If you try to re-thread a 25mm steerer, not only are the threads going to be undersize but the pitch will be off:
24 TPI = .0416 pitch = 1.058mm pitch.
1mm threads = .0394 pitch.
After about 4 or 5 turns the adjustable top race will reach a point of interference. A ham fisted, hammer mechanic can force it but the headset will always come loose from use.
There is nothing magic about metric threads. The whole world except for 3 countries, the US, the African nation of Liberia, and Myanmar (formerly Burma) use metric standard threads.
The Japanese bicycle industry adopted British Standard Bicycle Threads a long time ago. Because of the influence of Japanese bike makers (now Taiwan and China) the ISO (International Standards Organization) adopted British threads for bicycle headsets, bottom brackets and freewheel threads.
There are a number of PROPERLY functioning solutions for replacing the headset in your Motobecane Mirage. The first obstacle is finding a headset with a usable stack height.
You need to measure tour fork and frame to determine the stack height capacity:
Most European lower priced bikes used headset with a stack height of ~30mm to 33mm. VeloORANGE headsets have a 41mm stack height which is too high for all but the top pro model frames.
A quick search on eBay for "French Headset" came up with 84 items. Mike Fraysse always has these replacement French headsets for $24.99 USD:
Vintage Stronglight French 25x1 Peugeot Motobecane Bicycle Headset | eBay
Most French headsets had 27mm fork crown races whereas most inch size headsets (British/Japanese/ISO/Italian, etc.) had 26.4mm or 26.5mm crown races. 27mm fork crowns need to be cut down to use a crown 26.4mm crown race.
There is another alternative to replacing the whole headset. The fork crown race and lower headset cup are usually the only parts that wear out. Get one of the following Tange headsets and only use the bottom parts which have a classic look:
You can get a Tange Passage inch size headset with a 27mm crown race for under $20.00 USD. They have a 30mm stack height and can be found with a 27mm crown race:
The Tange Levin CDS headsets have a 33mm stack height and are also available with 27mm crown races. They're very good quality for the money - $15.00-$30.00 USD plus they have precision ground through hardened races and a plastic dust seal.
BTW, Tange made metric threaded forks both as replacements and OEM forks for French bike makers.
Easy fixes to simple problems...
verktyg