Abstract: A master jaw assembly for an engineering vise is provided by the invention. The master jaw assembly comprises a master jaw having a groove formed in its side wall face, and a detachable jaw having a dovetailed rib projecting from its side wall face so that the soft jaw may be attached to the master jaw by frontally inserting the dovetailed rib into the groove with minimal loosening of the hold-down nuts in the master jaw. A special hold-down nut used to secure the dovetailed rib of the detachable jaw inside the groove of the master jaw has a substantially longer base edge compared with its top edge to provide increased securing force. The master jaw also features an adjustable reference stop plate attached to the end of the master jaw that may be moved to its engagement position when needed to engage the edge of or interior niche in the detachable jaw to ensure that the detachable jaw is attached to the master jaw in precisely accurate alignment on a repeatable basis.