An analytical model has been developed to evaluate displacer gap losses in the clearance between the displacer and the cylinder in both β and γ configurations of Stirling engines. Displacer gap losses are the sum of the "shuttle heat transfer" and the "enthalpy pumping". The present model takes into account the pressure gradient in the gap, the gas compressibility and real gas effect. Gas velocity and temperature distributions in the displacer, in the gap and in the cylinder were determined by solving momentum and energy balances in a concentric tubes geometry. Our model is then introduced in a whole engine model and the effect of the clearance thickness and engine's speed on total displacer gap losses are investigated. Novel tendencies of the solution are observed and new ways for optimization are demonstrated.