Sure, 7mm sounds right, but how can "a trip to a hardware store and 3 LBSs yielded no dice?" It's an allen wrench for goodness sake.
(And ya, I edited my previous post 'cause I was being stupid.. *sigh*)
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FB4K - Every October we wrench on donated bikes. Every December, a few thousand kids get bikes for Christmas. For many, it is their first bike, ever. Every bike, new and used, was donated, built, cleaned and repaired. Check us out on FaceBook: FB4K.
Disclaimer: 99% of what I know about cycling I learned on BF. That would make, ummm, 1% experience. And a lot of posts.