The OP hasn't posted since sometime in '07 so don't expect an answer from him anytime soon. I think PastorBob has the best idea but I can't see why the size would be different. Since you can usually salvage the nut and spring I just get my screws at the True Value.
Sorry this has to be added.
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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