An external combustion engine containing a working fluid comprising: a burner element for heating the working fluid of the engine; at least one heater head defining a working space containing the working fluid; at least one piston cylinder containing a piston for compressing the working fluid; a cooler for cooling the working fluid; a crankcase comprising; a crankshaft for producing an engine output; a rocking beam rotating about a rocker pivot for driving the crankshaft; a piston rod connected to the piston; a rocking beam driven by the piston rod; and a connecting rod connected at a first end to the rocking beam and at a second end to a crankshaft to convert rotary motion of the rocking beam to rotary motion of the crankshaft; an airlock space separating the crankcase and the working space; and a piston rod seal unit wherein the external combustion engine is characterized in the piston rod seal comprising: a cylinder-gland housing between the working space and the airlock space, the cylinder gland housing configured to receive the piston rod; a cylinder gland; and at least one floating rod seal assembly mounted in the cylinder gland, the floating rod seal assembly comprising: a floating bushing configured to move axially and radially within the housing and disposed coaxially around the piston rod; a rod seal configured to seal the outside diameter of the piston rod relative to an inside surface of the floating bushing; and at least one stationary bushing fixed within the housing that may form a seal with the floating bushing to the axial flow of fluid in the presence of a pressure difference between the working space and the airlock space. a burner element for heating the working fluid of the engine; at least one heater head defining a working space containing the working fluid; at least one piston cylinder containing a piston for compressing the working fluid; a cooler for cooling the working fluid; a crankcase comprising; a crankshaft for producing an engine output; a rocking beam rotating about a rocker pivot for driving the crankshaft; a piston rod connected to the piston; a rocking beam driven by the piston rod; and a connecting rod connected at a first end to the rocking beam and at a second end to a crankshaft to convert rotary motion of the rocking beam to rotary motion of the crankshaft; a crankshaft for producing an engine output; a rocking beam rotating about a rocker pivot for driving the crankshaft; a piston rod connected to the piston; a rocking beam driven by the piston rod; and a connecting rod connected at a first end to the rocking beam and at a second end to a crankshaft to convert rotary motion of the rocking beam to rotary motion of the crankshaft; an airlock space separating the crankcase and the working space; and a piston rod seal unit wherein the external combustion engine is characterized in the piston rod seal comprising: a cylinder-gland housing between the working space and the airlock space, the cylinder gland housing configured to receive the piston rod; a cylinder gland; and at least one floating rod seal assembly mounted in the cylinder gland, the floating rod seal assembly comprising: a floating bushing configured to move axially and radially within the housing and disposed coaxially around the piston rod; a rod seal configured to seal the outside diameter of the piston rod relative to an inside surface of the floating bushing; and at least one stationary bushing fixed within the housing that may form a seal with the floating bushing to the axial flow of fluid in the presence of a pressure difference between the working space and the airlock space. a cylinder-gland housing between the working space and the airlock space, the cylinder gland housing configured to receive the piston rod; a cylinder gland; and at least one floating rod seal assembly mounted in the cylinder gland, the floating rod seal assembly comprising: a floating bushing configured to move axially and radially within the housing and disposed coaxially around the piston rod; a rod seal configured to seal the outside diameter of the piston rod relative to an inside surface of the floating bushing; and at least one stationary bushing fixed within the housing that may form a seal with the floating bushing to the axial flow of fluid in the presence of a pressure difference between the working space and the airlock space. a floating bushing configured to move axially and radially within the housing and disposed coaxially around the piston rod; a rod seal configured to seal the outside diameter of the piston rod relative to an inside surface of the floating bushing; and at least one stationary bushing fixed within the housing that may form a seal with the floating bushing to the axial flow of fluid in the presence of a pressure difference between the working space and the airlock space.