I've never seen half sizes above 5.5 mm. In fact, even 7 mm is not that common, generally; many hex-key sets don't include it. Could it actually have been an English 1/4" cap? Sometimes the tolerances on the bolts cause them to be a little oversize.
On the other hand, odd metric sizes are used occasionally. For example, Campagnolo delta brakes have a 3.5 mm fixing bolt for the cable, which can be difficult to find. However, an English 9/64" usually fits, if occasionally a bit tight.
Steve Maas Long Beach, California
brian blum wrote:
> My 70's Follis(not yours Jerry) came with a Belleri came with a 6.5 mm alle
> n bolt. Had a hard time tightening that stem. I remember riding home with a
> loose handlebar from the middle of nowhere becuase the 7 would not fit and
> the 6mm was too loose to secure it. A 1/4" did not fit that well and I did
> not carry SAE on bike rides. Was it common to specify half mm sizes? I thi
> nk some cleat bolts are 2.5 mm. Brian Blum in Berkeley
>
>
>
> Date: Tue, 03 Jan 2006 16:21:51 -0500
> From: Harvey M Sachs <sachshm@cox.net>
> To: dcwilson3@yahoo.com,
> Classic Rendezvous <classicrendezvous@bikelist.org>
> Subject: [CR]The Zen of 7 mm Hex Key stems
> Message-ID: <43BAEAEF.7070406@cox.net>
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> Reply-To: hsachs@alumni.rice.edu
> Message: 6
>
> Don Wilson's search for a 7 mm. hex key took me back to a night in the
> early 60s. Rice U, Houston, we raced some, and decent bikes were coming
> to campus (but don't ask about decent students. :-) ). I remember the
> night we rassled with a Frejus or Legnano which had a 7 mm. stem. There
> was nothing we could find that night that would loosen it. I finally
> used a 6 or some near-equivalent SAE, wedging it with a screwdriver
> blade on one side, and we got it freed up.
>
> But, my sense is that 7 mm was a pretty rare size even back then, or am
> I wrong? Does anyone know the purported advantages of the 7 over a 6?
> Yeah, the bolt is a bit lighter because of the bigger hex, but the
> wrench is heavier. Or was it just to be different?
>
> Clearly, this is not a critical question in the evolution of the bike,
> but I bet there are some good yarns out there...
>
> harvey sachs
> mcLean va
> -------