Piston pin bore and method of finishing

An aluminum piston comprising a piston head, a longitudinal axis, an outer peripheral surface, a hollowed center region for receiving a connecting rod, and a first piston pin bore extending through at least a portion of said piston in a direction transverse to said longitudinal axis and adapted to at least partially contain a piston pin, said piston pin adapted to be pivotally connected to said connecting rod, said bore comprising an outer end portion adjacent said outer peripheral surface, an inner end portion adjacent said hollowed center region and a center portion between said outer and inner end portions, each of said center and end portions having a generally circular cross-section, said bore being substantially smoothly curved going from said outer end portion to said inner end portion, the improvement wherein the diameters of each of said outer and inner end portions of said bore are substantially equal and are from about 0.0004 inches to about 0.0008 inches larger than the diameter of said center portion, whereby said bore has a smoothly curved hourglass configuration.