Below is a list of Lag Bolts manufacturers from San Marcos, California. A lag bolt is a mechanical fastener with a threaded body that is driven through two adjacent surfaces for the purpose of adjoining them securely. Also known as a lag screw, this piece of hardware is most often used to link wooden elements with other wooden parts or concrete foundations. Lag bolts are generally large, heavy duty and often industrial bolts, though smaller variations are available for use in furniture and cabinetry industries.