You really just need a short machine screw of the correct diameter. You could either take the frame to the hardware store or find something of about the same diameter as the hole to take with you as a guide. I'd try and get stainless steel
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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