Hi,

There are a couple of things to be sure of before modeling a mechanism in commercial 3D FEM tools. The modeling and simulation process depends whether you are considering a rigid body simulation or a multibody dynamic simulation with flexible bodies.

If you have made a note of that, then you can just use a translation type joint between the two cylindrical surfaces i.e. outer surface of the piston and the internal surface of the cylinder. If you have all other constraints set up properly, then this should work.

If you are using ansys, you can consider fractional B.C. between surfaceA