A surgical instrument that has an elongate channel assembly configured to operably support a staple cartridge therein. An anvil assembly may be movably and detachably coupled to the elongate channel assembly. In various embodiments, the anvil assembly may have a pair of trunnions pivotally received in open-ended slots in the elongate channel assembly. The trunnions may be retained in the opened ended slots by a selectively movable trunnion retention bar or by a closure member configured to apply a closing motion to the anvil. In other embodiments, a knife assembly may be supported within the elongate channel assembly for axial travel therein. The knife assembly may be provided with a detachable pin that is configured to ride in a slot in the anvil as the knife assembly is driven in the elongate channel assembly to draw the anvil assembly toward the elongate channel assembly. The pin is configured to be detached from the knife assembly to enable the anvil assembly to be detached therefrom.