A surgical instrument, comprising: an elongate channel configured to operably support a surgical staple cartridge therein; an anvil comprising an anvil body including a staple forming undersurface and an anvil mounting portion; wherein the anvil is pivotally coupled to a proximal end of the elongate channel, the anvil pivotal about a discrete, non-movable anvil axis defined by said elongate channel; a firing member configured for axial travel within the elongate channel in response to an application of firing motions thereto, the firing member being configured to movably engage the anvil and the elongate channel to space the anvil relative to the elongate channel at a desired spacing as the firing member is axially driven through the elongate channel; and a closure member configured to move the anvil between an open position and closed positions relative to the elongate channel upon application of closure motions to the closure member; characterised in that the anvil mounting portion comprises: a proximally protruding first anvil arm including an inwardly extending first anvil trunnion; and a proximally protruding second anvil arm spaced from the first anvil arm and including an inwardly extending second anvil trunnion spaced from the first anvil trunnion, said first anvil trunnion pivotally supported in a first trunnion hole in a first proximal end portion of the elongate channel and the second anvil trunnion pivotally supported in a second trunnion hole in a second proximal end portion of the elongate channel, the first and second trunnion holes coaxially aligned with each other on the anvil axis. an elongate channel configured to operably support a surgical staple cartridge therein; an anvil comprising an anvil body including a staple forming undersurface and an anvil mounting portion; wherein the anvil is pivotally coupled to a proximal end of the elongate channel, the anvil pivotal about a discrete, non-movable anvil axis defined by said elongate channel; a firing member configured for axial travel within the elongate channel in response to an application of firing motions thereto, the firing member being configured to movably engage the anvil and the elongate channel to space the anvil relative to the elongate channel at a desired spacing as the firing member is axially driven through the elongate channel; and a closure member configured to move the anvil between an open position and closed positions relative to the elongate channel upon application of closure motions to the closure member; characterised in that the anvil mounting portion comprises: a proximally protruding first anvil arm including an inwardly extending first anvil trunnion; and a proximally protruding second anvil arm spaced from the first anvil arm and including an inwardly extending second anvil trunnion spaced from the first anvil trunnion, said first anvil trunnion pivotally supported in a first trunnion hole in a first proximal end portion of the elongate channel and the second anvil trunnion pivotally supported in a second trunnion hole in a second proximal end portion of the elongate channel, the first and second trunnion holes coaxially aligned with each other on the anvil axis. a proximally protruding first anvil arm including an inwardly extending first anvil trunnion; and a proximally protruding second anvil arm spaced from the first anvil arm and including an inwardly extending second anvil trunnion spaced from the first anvil trunnion, said first anvil trunnion pivotally supported in a first trunnion hole in a first proximal end portion of the elongate channel and the second anvil trunnion pivotally supported in a second trunnion hole in a second proximal end portion of the elongate channel, the first and second trunnion holes coaxially aligned with each other on the anvil axis.