Abstract: The piston rod of the tandem-piston engine was modified to include a crosshead that slides in a crosshead guide machined into the lower part of the charger piston for better lateral support. The lower surface of the motor piston was changed to a simple conical surface allowing increased vertical thickness of the central region with a thinner outer region. A ceramic piston crown was clamped in compression by a piston bezel to the motor-piston base at a ledge with a conical upper surface having its vertex at the center of its base so thermal expansion will effect a sliding contact. A piston ring was used at the upper edge of the motor piston so the uppermost cylinder wall will be polished to prevent heat absorption. An intake port was given a flame arresting diffuser; and its entrance was blocked by the piston valve to prevent ignition in the valve cylinder.