A chainsaw guide bar adapted for making small radius curvilinear cuts comprising: an elongated guide bar body defining a longitudinal axis, a top guide rail edge and a lower guide rail edge; a proximal broadened mounting end configured for mounting to a power drive unit; a distal working end having a tip; an intermediate cutting portion having an axial length and extending from the working end towards the mounting end, the cutting portion defining a portion of the top and lower guide rails edges (508, 509) for making upwards and downwards cuts in a workpiece; a transition portion defined between the cutting portion and broadened mounting end, the transition portion forming a downwardly inclined curvilinear compound shaped section of the top guide rail edge, the curvilinear compound shaped section comprising an arcuately shaped convex edge surface and a contiguous adjacent arcuately shaped concave edge surface forming a rounded smooth transition into the upper active cutting edge of the cutting portion; a peripheral guide rail slot longitudinally extending along both top and lower edges of the guide bar body from the distal working end to the proximal broadened mounting end, the guide rail slot configured to slidably engage a circulating saw chain, the saw chain including a plurality of cutting links pivotally connected to drive links slidably engaged in the guide rail slot; and an idler sprocket mechanism mounted above the guide bar body proximate to the concave arcuately shaped edge surface of the transition portion, the sprocket mechanism positioned outboard of the saw chain, the sprocket mechanism engaging the saw chain and being operative to keep the guide tangs of the chain engaged with the guide rail slot in the arcuately shaped concave edge surface of the transition portion as the saw chain circulates; wherein the saw chain is fully exposed forward of the transition portion and operable for cutting in both upward and downward directions with the cutting portion of the guide bar. an elongated guide bar body defining a longitudinal axis, a top guide rail edge and a lower guide rail edge; a proximal broadened mounting end configured for mounting to a power drive unit; a distal working end having a tip; an intermediate cutting portion having an axial length and extending from the working end towards the mounting end, the cutting portion defining a portion of the top and lower guide rails edges (508, 509) for making upwards and downwards cuts in a workpiece; a transition portion defined between the cutting portion and broadened mounting end, the transition portion forming a downwardly inclined curvilinear compound shaped section of the top guide rail edge, the curvilinear compound shaped section comprising an arcuately shaped convex edge surface and a contiguous adjacent arcuately shaped concave edge surface forming a rounded smooth transition into the upper active cutting edge of the cutting portion; a peripheral guide rail slot longitudinally extending along both top and lower edges of the guide bar body from the distal working end to the proximal broadened mounting end, the guide rail slot configured to slidably engage a circulating saw chain, the saw chain including a plurality of cutting links pivotally connected to drive links slidably engaged in the guide rail slot; and an idler sprocket mechanism mounted above the guide bar body proximate to the concave arcuately shaped edge surface of the transition portion, the sprocket mechanism positioned outboard of the saw chain, the sprocket mechanism engaging the saw chain and being operative to keep the guide tangs of the chain engaged with the guide rail slot in the arcuately shaped concave edge surface of the transition portion as the saw chain circulates; wherein the saw chain is fully exposed forward of the transition portion and operable for cutting in both upward and downward directions with the cutting portion of the guide bar.