On the rarity of the 3.5 mm allen wrench (0.138") can't you use a 9/64" (0.140") one?
The wrench isn't that rare you just need to think outside the bike shop.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk