Abstract: Design Improvements are disclosed which enhance the migrating combustion chamber engine's ability to achieve improved performance, obtain higher durability and cost less to manufacture. These include strip seals between the combustion chamber member and orbiting piston which are adapted to respond to the pressure of combustion to increase contact pressure and improve retention of the gases in the combustion chambers as well as improved porting located in at least one power block sidewall and cooperating with the migrating combustion chamber to convey hot combustion gasses from a combustion chamber to a corresponding secondary expansion chamber. The combustion chamber member may be formed of two reciprocable piston portions and a pair of separate alloy steel connecting bars coupling the piston portions together. The connecting bars made of a low thermal conductivity material to remain hot and aid in fuel evaporation. A one piece counterweight hub provides all required counterweights.