This article shows the development of a twin cylinder alpha type Stirling engine model, as well as the redesign of a prototype, its manufacture and preliminary results. The mechanical model is coupled with a thermodynamic model which allows an analysis and comparison of the theoretical and experimental behavior of the machine and a better understanding of the whole process. On the other side, a prototype redesign is presented. Thermal circuit was improved in order to eliminate losses, reduce dead volumes and improve heat transfer to the working fluid. Modifications of the seal system and the pistons design are included as well. An equation for the sizing of the flywheel is developed and finally the results obtained in tests between the new and the previous design are compared. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.