Here's my guess. Cinelli didn't use the 7mm opening to be different. In fact, TTT used this size as well. I believe they felt that you couldn't generate enough torque with a 6mm hex key to properly tighten the quill's bolt. Remember, this was not so long after the time when all stems used an ordinary hex headed bolt head. (10mm to 12mm if I recall correctly.)
So I suppose that, eventually, they realized 1) you could generate sufficient torque with a 6mm hex key 2) any manufacturer of fasteners could supply an elongated 6mm hex cap screw, whereas the 7mm size required extra setup time 3) 7mm hex keys weren't that simple to find.
Like everyone else, I struggled to find a 7mm hex key in those days. It seemed as though the few shops that had one didn't have any extras to sell. I managed to order one (made by Hazet) from a German sports car parts specialist. It eventually "disappeared". But now I have an original Cinelli version, one from Snap-On and several from Sears. In fact, I think I now have more of those 7mm hex keys than I have stems that use them, but I don't ever want to be caught without one when I need it!
Fred Rednor - Arlington, Virginia (USA)
> Don Wilson's search for a 7 mm. hex key took me back to a
\r?\n> night in the
\r?\n> early 60s. Rice U, Houston, we raced some, and decent bikes
\r?\n> were coming
\r?\n> to campus (but don't ask about decent students. :-) ). I
\r?\n> remember the
\r?\n> night we rassled with a Frejus or Legnano which had a 7 mm.
\r?\n> stem. There
\r?\n> was nothing we could find that night that would loosen it. I
\r?\n> used a 6 or some near-equivalent SAE, wedging it with a
\r?\n> blade on one side, and we got it freed up.
\r?\n> But, my sense is that 7 mm was a pretty rare size even back
\r?\n> then, or am
\r?\n> I wrong? Does anyone know the purported advantages of the 7
\r?\n> over a 6?
\r?\n> Yeah, the bolt is a bit lighter because of the bigger hex,
\r?\n> but the
\r?\n> wrench is heavier. Or was it just to be different?
\r?\n> Clearly, this is not a critical question in the evolution of
\r?\n> the bike,
\r?\n> but I bet there are some good yarns out there...
\r?\n> harvey sachs
\r?\n> mcLean va