"Point at" is recommended by BL Render as well; she describes a piston in her book (Secrets of Figure Creation with Poser 5).

If for some reason you don't want to do it that way, you can use ERC.  The problem is, the rotations of the piston parts are not linear equivalents of the rotation of the joint.  For a small degree of motion, close enough.  For larger degrees of motion, you need ERC math....