A surgical instrument (10) comprising: a handle assembly (12) a body (14) extending distally from the handle assembly, the body defining a first longitudinal axis a tool assembly (17) pivotally supported on a distal end of the body defining the tool assembly a second longitudinal axis, being pivotable tool assembly between a non-articulated position in which the first longitudinal axis is aligned with the second longitudinal axis and at least one articulated position in which the second longitudinal axis is angled relative to the first longitudinal axis and a linkage mechanism (30) including a receptacle (436) positioned adjacent the handle assembly, a main shaft (432) having a base portion (476), the main shaft rotatably supported within the receptacle being, a lower clutch (430) fixedly positioned within the receptacle, the lower clutch a knurled portion (444) and includes a portion of striae (450) and located around the main shaft, an upper clutch slidably positioned about the shaft main, the upper clutch rotationally fixed to the mainshaft such that the rotation of the main shaft affects rotation of the upper clutch, including the upper clutch at least one projection positioned to engage the grooves of the lower clutch to releasably retain releasably the main shaft in a fixed position rotationally and a joint rod (123) having a proximal end operatively connected to the base portion of the main shaft and a distal end operatively connected to the tool assembly, wherein the main shaft is rotatable to move the connecting rod articulation in order to affect movement of the tool assembly between the position unarticulated and at least one articulated position wherein a radial angle between a first pair of adjacent grooves (450a, 450b) is defined as a first angle (α1), a radial angle between a second pair of adjacent grooves (450b, 450c) is defined as a second angle (α2 ), and characterized in that a radial angle between a third pair of adjacent grooves (450d, 450e) is
