A piston assembly for heavy duty diesel engine applications comprises a piston having pin bosses formed with pin bores for the accommodations of a wrist pin. The wrist pin is fabricated of steel and is extendable through the small end bore of a steel connecting rod and the bores of the piston body for coupling the piston body to the connecting rod. In lieu of a conventional Cu—Pb—Sn bushing usually pressed into the small end bore, a coating of manganese phosphate is applied to either the pin or small end bore of the connecting rod and pin bosses to provide the necessary tribological properties therebetween. The porous coating absorbs and entraps lubricating oil, creating a still lubricant squeeze film between the steel running surfaces under heavy load.