A method for reducing the incidence of piston pin failure by providing a retainer assembly for piston pin plugs such that the plugs cannot be removed or shaken lose during operation. A plastic rod is inserted into the piston pin, and the rod is ultrasonically welded to the stems of mushroom-shaped plastic piston plugs such that the plugs cover the open ends of the pin and are held in place by the cooperation of structure between the plugs and the rod. In an alternate embodiment, the stem of each pin plug is hollow and a respective internally threaded brass insert is disposed in the stem and ultrasonically welded therein. An externally threaded rod is then engaged with the two internally threaded inserts to hold the plugs on the piston pin.