Hi - I have an old frame that I'm trying to build up. It's a Motobecane Super Mirage, most likely from the 70s; it says it's made in France. I'm trying to figure out what size wheels it needs, which I naively thought would be a simple question...

I tried swapping the front wheel from another bike (Mavic CXP22 wheels, supposedly 700c), but the radius is a bit too small, by maybe 1cm; enough that the brakes wouldn't reach the rims.

Did bikes from this time use a different standard for wheel size? Or do I just need to find brakes with a longer reach?

If you already have a set of CXP22 wheels, front and rear, then get brakes with longer reach. Those wheels will be better in the long run. BUT, yes, your bike was made for 27 inch wheels which I have on my Motobecane and they are fine but I already had the wheels.

-----

the difference in size is eight mm's of diameter, or four mm's in radius

sounds like your example one produced for export to the english speaking world where twenty-sevens were the norm


-----

This raises an interesting question: did Motobecane (and other European manufacturers for that matter) actually build different frames and forks for for the English-speaking part of the world, or did they just use different brake calipers to deal with the 4mm reach difference?

Back when the OP's Super Mirage was made it may have been spec'd and built for Ben Lawee who was the main importer of this Motobecane brand to the USA, so frames made for this market would have been built to fit the wheels he specified.
and in mid-1970s the catalogs say all models from the Nobly to Grand Record came with 27" rims and tires, only the 2 top-of-range models (Le Champion and Team Champion) came with 700 C tubulars.
However I own an approx 1976 GR and it has the presumed OEM Weinmann centerpull calipers which reach my 700 C clincher rims with no problem, and this bike came to me with vintage tubbie rims laced to Normandy Luxe hubs which tells me that "specifications are subject to change"!

EDIT: I was looking at just one (1976) USA Moto catalog and assumed due to bad memory that the specs were consistent for at least a 3-year span, but clearly there were a LOT of "running changes" both some that got published and some did not.

If you already have brake calipers and they reach the 27" rims, it is possible to file the slots in the calipers to be longer so you can slide the brake pads further down so they will reach 700C rims. I did this on my Batavus. Eyeball the calipers carefully and measure to determine if you can remove about half the meat remaining.

+ 1 on this. I recently had to file the slots on centerpull brakes to make them work with 700c wheels for a bike designed for 27 inch wheels.

Thanks guys! Always amazed at the depth of expertise here.