A general analysis has been carried out, dealing with gas spring operation, with particular reference to free-piston Stirling engine applications. Previous work, based on the assumption that losses due to gas leakage are negligible, provided an energetic model together with an experimental method suitable for the evaluation of the convection heat transfer coefficient in the gas spring chamber. As a second step, this paper deals with a general energetic model for the evaluation of gas flow through the clearances between the piston and the cylinder of the spring. In particular, the paper includes a description of a test bench suitable for the validation of the model, together with experimental investigations carried out in order to determine the parameters of the adopted energetic model.