http://www.cbp.gov/linkhandler/cgov/trade/legal/informed_compliance_pubs/icp013.ctt/icp013.pdf The Federal Government's official document on distinguishing bolts from screws.
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http://www.cbp.gov/linkhandler/cgov/trade/legal/informed_compliance_pubs/icp013.ctt/icp013.pdf The Federal Government's official document on distinguishing bolts from screws.