Abstract: An expansion engine incorporates various structural features in which all cold seals are stationary, and only warm seals move. Moving parts which are subject to cryogenic temperatures are designed with gas bearings, while tight tolerances and material choices inhibit wear and steady state heat loss. Numerous other features include structural design to relieve stresses, combined fabrication of key parts, initial bias of the inlet and outlet valves which contract on cooldown to desired alignment, and a gas-purged upper seal housing, eliminate various sources of expansion engine failure and heat loss during operation and result in high reliability and thermal efficiency.