Dynamic Meshing Problem
 

I'm currently in the learning stages for dynamic meshing of a 3D in cylinder model. I made a a simple 2d box mesh in ICEM CFD and imported it into Fluent, with one side moving like a piston. The problem is that the meshes are quite different during the different strokes of the piston. During the return stroke, the mesh quality is highly irregular. I'm attaching pics with this to make the problem clearer. If anyone could help.