An external combustion engine is disclosed. The external combustion engine includes a working fluid and a burner element, at least one heater head defining a working space, at least one piston cylinder containing a piston, a cooler, a crankcase including a crankshaft for producing an engine output, a rocking beam, 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. The external combustion engine also includes an airlock space separating the crankcase and the working space for maintaining a pressure differential between the crankcase housing and the working space housing and an airlock pressure regulator connected between the crankcase and one of the airlock space and working space.