[RobotDoktor]

First ensure that the piston can be properly actuated in its own subassembly. Ones downloaded from the manufacturer should do this. Then make sure it is properly defined in the full assembly. Finally change the solve as property of the piston sub assembly from lightweight to resolved. Look that up in the Solidworks help page for more details. Once you do this you should be able to use the normal motion study actions on the piston.

Hope that helps. Otherwise I may be able to post better instructions when I'm on my workstation later.

Edit: right click on the piston sub-assembly and go to component properties. Ensure the suppression state is set to resolved. If it is not change it and save. The change the "solve as" to flexible. Any parts that can move/actuate in the sub-assembly should now be able to do the same in the full assembly.