An assembly for translating linear motion of a linearly moveable member along a longitudinal axis to rotary motion of a rotatable shaft about a first axis, and vice-versa, said linearly moveable member being movable between a first dead centre position and a second dead centre position along said longitudinal axis, said assembly comprising: a yoke structure associated with said linearly movable member and movable therewith, said yoke structure including at least one inner arced surface (28, 62) having a radius measured from the first axis at a shaft rotation angle between 0 and 90 degrees; a leverage means associated with said shaft and rotatable with said shaft about said first axis, an outer peripheral edge of said leverage means including at least one arced contact surface; wherein dimensioning of the leverage means and the yoke structure is such that the inner arced surface (28, 62) of the yoke structure and the arced contact surface of the leverage means contact each other during a stroke of the linearly movable member; wherein, from a 0 degree shaft rotation angle to an approximate 90 degree shaft rotation angle, contact between the yoke structure and the leverage means occurs at a first contact point along the leverage means arced contact surface, wherein the first contact point lies on a straight line which passes through the first axis and a centre of curvature of the at least one arced contact surface; and wherein the arced contact surface of the leverage means is dimensioned such that from an approximate 90 degree shaft rotation angle to an approximate 180 degree shaft rotation angle the first contact point between the arced contact surface of the leverage means and the inner arced surface of the yoke structure moves in a single direction along the arced surface of the leverage means to cause a rocking motion between the two surfaces; and wherein the yoke structure inner surface includes at least one linear surface (56, 58) and said leverage means is adapted to contact said linear surface (56, 58) between an approximate 0 degree shaft rotation angle and an approximate 180 degree shaft rotation angle to thereby provide a second contact point between the leverage means and the yoke structure. a yoke structure associated with said linearly movable member and movable therewith, said yoke structure including at least one inner arced surface (28, 62) having a radius measured from the first axis at a shaft rotation angle between 0 and 90 degrees; a leverage means associated with said shaft and rotatable with said shaft about said first axis, an outer peripheral edge of said leverage means including at least one arced contact surface; wherein dimensioning of the leverage means and the yoke structure is such that the inner arced surface (28, 62) of the yoke structure and the arced contact surface of the leverage means contact each other during a stroke of the linearly movable member; wherein, from a 0 degree shaft rotation angle to an approximate 90 degree shaft rotation angle, contact between the yoke structure and the leverage means occurs at a first contact point along the leverage means arced contact surface, wherein the first contact point lies on a straight line which passes through the first axis and a centre of curvature of the at least one arced contact surface; and wherein the arced contact surface of the leverage means is dimensioned such that from an approximate 90 degree shaft rotation angle to an approximate 180 degree shaft rotation angle the first contact point between the arced contact surface of the leverage means and the inner arced surface of the yoke structure moves in a single direction along the arced surface of the leverage means to cause a rocking motion between the two surfaces; and wherein the yoke structure inner surface includes at least one linear surface (56, 58) and said leverage means is adapted to contact said linear surface (56, 58) between an approximate 0 degree shaft rotation angle and an approximate 180 degree shaft rotation angle to thereby provide a second contact point between the leverage means and the yoke structure.