In an opposed piston engine, a pair of pistons are disposed in opposition in the bore of a cylinder. The cylinder includes first liquid coolant grooves having a first cooling capacity to cool a portion of the cylinder extending from a central portion toward an exhaust port, and second liquid coolant grooves having a second cooling capacity, less than the first cooling capacity, to cool a portion of the cylinder extending from the central portion toward an inlet port. Each piston includes a cylindrical skirt with a crown and an open end opposite the crown, a piston rod with a bore, a first end attached to a back surface of the crown, and a second end extending through the open end of the skirt, a radial array of liquid coolant flow passages in communication with the bore and disposed between the first end and the back surface of the crown, and a single wristpin retained on the second end section of the piston rod and positioned externally to the piston.